A Prince George's Circuit Court judge set aside a jury-imposed death sentence yesterday for Reuben Jackson Jr. and sentenced the convicted murderer to life plus 30 years in prison.

The decision by Judge Arthur M. Ahalt came after three days of testimony during a postconviction hearing and weeks of negotiations between Jackson's attorneys and the Prince George's state's attorney's office.

The Southeast Washington man had been the first person sentenced to death by a jury in Prince George's County since 1978, when capital punishment was reinstated in Maryland.

Jackson was sentenced in October to die in the gas chamber for the murder, kidnaping and armed robbery of Joanne Grossnickle, a 22-year-old Frederick County native who had recently moved to Washington to work as a lobbyist with the Church of the Brethren.

Grossnickle's body, stabbed several times, was found Sept. 9, 1984, in a grassy area in Cottage City. She had disappeared six days earlier.

Jackson was arrested Sept. 10, 1984, by District of Columbia police, who spotted him driving Grossnickle's car, which contained several items missing from her Silver Spring apartment.

After his conviction Jackson asked Ahalt for a new trial, saying that a juror had failed truthfully to answer questions during jury selection. Ahalt turned down Jackson's request for a new trial but granted a new sentencing after both sides went to him with an agreement.

Assistant State's Attorney Bond Rhue, who prosecuted Jackson, said yesterday that the new sentence would put the state in a stronger position when Jackson appeals his conviction and sentence.

"I look like I fool when I say 'no comment,' but this case may be up for appeal, and I don't want to say anything that might strengthen the defendant's case," Rhue said.

Byron Grossnickle, the victim's father, said he "was in agreement" with the state's attorney's office. "The alternative might have been Jackson walking free," he said.

Jackson, 40, of 3459 Minnesota Ave. SE, told Ahalt he agreed with the new sentence but was not admitting guilt.

"I'm innocent of what the state has put on me," Jackson said. "At no time have I hurt anyone that wasn't going to hurt me first . . . that I know of."

Jackson will serve at least 35 years of the sentence before he will be eligible for parole, a state's attorney's office spokesman said yesterday.