With a smile on his lips and tears in his eyes, the 38th President of the United States yesterday because citizen Gerald Rudolph Ford.

Flying out to California on the back-up plane to Air Force One, a shirt-sleeved Ford acknowledged to reporters that the transfer of power he had accomplished with such grace had not been easy for him.

"I don't deny that I got a little sentimental, but I tried not to expose it, to keep it within myself," Ford said. "We're had 2 1/2 great years and you can't help but be a little emotional and sentimental. I'm human like anybody else."

The most difficult moment of the day, Ford said, came in the opening moment of President Carter's inaugural address.

"I want to thank my predecessor for all he has done to heal our land," Carter said.

The inaugural audience applauded, and Ford, nearly overcome, bit his lip to keep from crying.

He was close to tears, too, as was his wife, Betty, when the Fords and Vice president Rockefeller and his wife were honored by a military review at Andrews Air Force Base, where the Fords departed for a brief golfing vacation in Monterey, Calif. This time the sentiment came when the band played "God Bless America" and the Fords walked up the ramp of the U.S. Air Force 707 emblazoned with the familiar presidential seal.

Ford looked straight ahead, fighting his emotions. Betty walked proudly up the ramp carrying a bouquet of red roses that had been given her by the crew of the helicopter that flew the Fords from the Capitol to Andrews. At the top of the ramp both Fords turned and waved, long and lovingly, at a small farewell crowd where a child's sign proclaimed, "God bless you."

There was another time earlier in the day, which Ford did not tell reporters about, when he also had struggled to keep emotional control. This occurred on his last visit to the Oval Office, here Ford signed a few last papers and permitted two photographers to take a final picture.

Leaving the office, Ford paused in an adjoining room to say good-bye to his secretary, Nell Yates. He kissed her, and she cried. Ford walked silently out through the reception room and joined his wife in the residential quarters of the White House for the last time.

Ford is by nature an optimistic man, but the happiness and optimism of this farewell day came not from him but from Betty. She said her mood was one of nostalgia, not sadness, and she was stirred by the view of Washington from the helicopter that carried them to Andrews.

Betty Ford said later that the only time she was emotionally upset during the day was when the Fords left the Capitol where "our whole married life has spent" and were taken to the helicopter. But she confided that she had experienced a "sence of relief" at having her husband and herself freed from the pressures of the White House.

"Now that your husband is unemployed, what do you plan to do with him?" a reporter asked.

"Take care of him," she said, giving Ford a big hug.

"I think I'd better leave," the former President said to laughter. And he did leave, allowing his wife to explain her happiness at no longer having to share him with the country.

In recent weeks Ford has hinted that he might run for office again someday. He made the same suggestion again yesterday, out of the ear-shot of Betty, who was uninterested in her husband's/becoming another John Quincy Adams, who resumed a House career after the presidency, or another Grover Cleveland who was elected President four years after being defeated as a presidential incumbent.

"Twenty-eight years is enough," she said with finality.

Later in the 5 1/2 hour flight, Ford said that he would not be an Adams, but he would not rule out another try at the presidency. He emphasized that he wanted very much to remain active in Republican political affairs and added, possibly with former challenger Ronald Reagan in mind: "I don't want anyone to preempt the Republican presidential position."

Ford is not a man to remain despondent for very long, and yesterday was no exception.

On board the airplane with several of his transition staff and his two golden retrievers, Liberty and Misty, Ford began to look forward to life beyond the presidency.

He drank two maritnis, ate a dinner of shrimp and tender prime rib and talked about the golf game he will play today will Arnold Palmer in the Bing Crosby pro-am tournament at Pebble Beach.

He and Betty, Ford said, can enjoy "the best of both worlds," by which he meant his presidency and an active retirement.

In the bleak days last November after his narrow election defeat, Ford second-guessed his loss and became despondent. Now, he already perceives a unique status of his presidency. Yesterday, in the hour of Carter's ascension, it was given to Gerald Ford, as it is to few men, to pronounce the epitaph of his own administration.

Speaking to 75 staff members at a final breakfast in the White House, Ford praised them and said:

"You all contributed to an administration which I think was good, and which history will treat kindly."