A Fairfax County deputy sheriff was convicted yesterday of stealing a county jail inmate's suede jacket last October and was sentenced to serve 12 months in the jail.

Fairfax General District Court Judge J. Mason Grove found Sgt. Frank Gibson, 41, of Lorton guilty of petit larceny, a misdemeanor that carries a maximum penalty of a year in jail and a $1,000 fine.

Gibson said he will appeal the verdict. He will remain on administrative leave from the sheriff's department with pay until the appeal is decided by the county Circuit Court.

Gibson was convicted of stealing a new suede jacket owned by a man who started serving 30 days in jail last October for failing to appear at a hearing. The jacket was placed in the jail property room along with the man's other belongings, according to testimony at the trial. At Gibson's request, a trusty removed it from the room and put it in the trunk of Gibson's car, according to testimony.

The trusty, Douglas McGilton, who has since been released from the Fairfax jail, testifed that he wrote to police about the jacket incident last month.

Gibson did not testify.

Before sentening, Fairfax Commonwealth's Attorney Robert F. Horan Jr. told the judge that Gibson had been convicted in U. S. Magistrate's Court in Alexandria last May of attempted larceny of government property. Gibson's attorney said Gibson was fined $25 in that incident.

Gibson is the second Fairfax sheriff's deputy in a month to be convicted of a misdemeanor. Last month chief jailer John O. Feehan pleaded guilty to using a prisoner to work outside the jail and was fined $25. Feehan is still on duty at the jail.

Prosecutor Horan is investigating the alleged use of jail inmates for private work outside the jail and allegations tht food meant for inmates has been consumed by sheriff's department employes.