Frank Mason Jr., 18, went to court yesterday to be sentenced for the felony murder of Gladys M. Werlich, 85, a Washington society matron who was fatally injured during an attempted street robbery last January.

Like many prisoners about to be sentenced, Mason had something to say, and he put it in the form of a letter to Judge Nicholas S. Nunzio, who was presiding in the D.C. Superior Court case.

"I can only start by letting you know that I am scared," Mason wrote. "I am not writing to ask for any favors because I realize I have done wrong."

The letter, which Nunzio read in open court, was one of several documents he had to consider. Another was a report from corrections officials who had studied Mason and recommended that he receive a 12-year term under the Youth Corrections Act.

Moreover, he had to consider the crime.Mason pleaded guilty last September to felony murder in Mrs. Werlich's death. He said he had knocked Mrs. Werlich to the sidewalk on Corcoran Street N.W. near her home at 1625 16th St. after a companion tried and failed to steal her pocketbook. Mrs. Werlich died of head injuries six days later.

Mason repeated these admissions earlier this month in the trial of Richey R. Weaver, 17, and Leroy Parker, 19, two other youths charged in the death. Parker was found guilty of second-degree murder and Weaver guilty of attempted robbery. A 13-year-old has been found guilty of felony murder - murder resulting from another crime, in this case attempted robbery - in juvenile court.

In his letter to Nunzio, Mason said: "I truly wish to mold a man, one that I can be proud to identify as being me. But, Sir, all these things seems like a dream. I seem to be lock in a tomb with just a beam of light showing represent(ing) the beauty that awaits me."

Nunzio a former prosecutor, was so impressed with the letter that he said he would keep it in his personal files.

Then he sentenced Mason to a maximum of six years to be served in the federal youth facility in Martinsburg, W. Va. There is no minimum sentence under the Youth Corrections Act.