Lt. Cmdr. Mary Ann Cronin, a Navy nurse, was convicted yesterday of driving while intoxicated in connection with the death of a 16-year-old Fairfax County youth and sentenced to 90 days in jail, a $500 fine and a six-month suspension of her driver's license.

Cronin's attorney immediately appealed the verdict by Fairfax General District Judge Robert M. Hurst to the Circuit Court, a move that voided the conviction and will allow Cronin a new trial in the higher court.

The attorney, Barry Murphy, claimed that prosecutors had unconstitutionally placed Cronin on trial twice for the same offense, since she was acquitted by a jury last month of involuntary manslaughter in the same incident, in which 16-year-old John Svec was hit by Cronin's car as she was driving home from a nurses' reception at which she had drunk six beers.

Cronin is a head surgical nurse at Bethesda Naval Hospital with a 12-year career in the Navy. If she is convicted at the Circuit Court level of driving while intoxicated, she could lose her military commission, according to naval authorities. Attorneys on both sides predicted that if Cronin is convicted, the case will be appealed to the Virginia Supreme Court.

Cronin, 34, wearing the white Navy nurse's uniform that she wore throughout the emotional two-day involuntary manslaughter trial in August, trembled as Hurst said, "I think the defendant is guilty."

The parents of John Svec, who was walking along an Annandale highway with friends when he was struck by Cronin's car on May 13, said "justice was served" by the conviction. The Svecs, along with representatives of Mothers Against Drunk Drivers, monitored yesterday's one-hour trial.

Assistant Commonwealth's Attorney Jack A. Robbins introduced the results of Breathalyzer tests that showed Cronin had a blood alcohol level of .14 percent shortly after the late night accident. Under Virignia law, a driver is considered intoxicated when the level reaches .10.

But defense attorney Murphy repeatedly protested the court proceedings, claiming: "This is the grossest form of jeopardy. . . . They prosecutors had the chance to prove DWI driving while intoxicated and the jury at last month's trial didn't buy it."

Robbins argued that the involuntary manslaughter and drunk driving trials involved separate issues. He said intoxication was only one of several issues the jury could have considered in determining whether Cronin was guilty of involuntary manslaughter, and that the acquittal in the involuntary manslaughter case did not preclude the possibililty that Cronin was drunk at the time.

"To say that she was already found not guilty of DWI. . . would require the court to speculate on why the jury found her not guilty of involuntary manslaughter," said Robbins.

Three Woodson High School students who were walking with Svec along the edge of Little River Turnpike near Northern Virginia Community College testified yesterday--as they had at the earlier trial--that they saw the reflection of headlights from Cronin's black Datsun as the car veered off the main roadway and on to the bus lane where they were walking.

"I heard the sound of John hitting the car," said George Pawlak.

Cronin did not testify yesterday. At her trial last month, she testified tearfully that she was tired and never saw Svec walking along the roadside before her car hit him.

Under Virginia law, the maximum penalty for a first-time conviction of driving while intoxicated is 12 months in jail and a $1,000 fine. In sentencing Cronin to a jail term, Judge Hurst imposed a 180-day sentence but suspended all but 90 days.