The unemployment rate held steady in August at 6 percent, its lowest level in nearly eight years, the government reported yesterday. But economists said it still was high by historical standards for this far into an economic expansion.

The number of Americans working, as measured by the Labor Department's household survey, swelled by a seasonally adjusted 354,000 to a record 113.1 million.

However, the civilian labor force, which includes those working and those actively looking for jobs, grew by a similar 351,000, leaving the August unemployment rate identical to the July rate and down from 6.8 percent a year ago.

The number of jobless workers fell last month to 7.2 million, down 3,000 from July and the lowest number since March 1980, the Bureau of Labor Statistics said.

During the past 15 months, the jobless rate gradually has edged down from 7.2 percent without a single upward movement. The last time the unemployment rate was lower was in November 1979, when it stood at 5.9 percent.

The recovery from the 1981-1982 recession now is 58 months old, tying the record set in 1975-1980 for the longest peacetime expansion.

The proportion of the population employed rose to a record 61.8 percent, up from 61.6 percent in July.

However, Janet L. Norwood, commissioner of the Bureau of Labor Statistics, told the Joint Economic Committee of Congress that the pace of employment growth, especially in factory jobs, is slower than during the previous 1975-1979 expansion.

Rudy Oswald, chief economist of the AFL-CIO, said that the 6 percent level "is clearly a big improvement from where we were earlier in this decade, but it is still relatively high compared with any recovery in the postwar period.

"The other question is what happens to those who are unemployed. In August, only 29 percent received some form of unemployment benefits," he said.

An alternate rate, which includes in the calculations 1.7 million uniformed members of the armed forces stationed in the United States, also remained unchanged in August at 5.9 percent.

Pointing to this alternate rate, White House spokesman Marlin Fitzwater said in Santa Barbara, Calif., that total employment rose by 370,000 workers. "The employment-to-population ratio rose to another record high of 62.2 percent," Fitzwater said. "Jobs are the best economic barometer there is, and the meter reads, 'Excellent.' "

Oswald said the Federal Reserve Board's decision to boost its discount rate from 5.5 percent to 6 percent, announced yesterday, would likely depress employment in construction and manufacturing.

But Jerry Jasinowski, chief economist of the National Association of Manufacturers, said the increase in the discount rate would have little effect on employment. This "is primarily a symbolic move confirming to the money markets that the Federal Reserve is going to maintain a fairly snug monetary policy," he said.

According to a separate Labor Department survey of public and private payrolls, released at the same time as the household survey, the economy has created nearly 2.5 million new jobs in the past year, 155,000 of them in August. Service jobs continued to account for most of the growth.

The largest portion of them were in business and health services, up 90,000 from July, and in finance, insurance and real estate, up 25,000. Employment in retail trade dropped by 23,000 in August, the first decrease of this year.

Manufacturing jobs also dropped, by 5,000, ending six months of steady growth.

Norwood said the economy has produced nearly 14 million jobs since November 1982, but only about 1 million have been in manufacturing, which "has still only regained about 45 percent of the jobs lost during 1981 and 1982."

The number of people working part time because they could not find full-time jobs dropped by nearly 250,000 in August. But the total, 5.26 million, was still nearly 80,000 more than in June. "These partially unemployed workers are disproportionately black and female," Norwood said.