An 88-year-old Catholic priest who sexually abused an alter boy in Melbourne in the 1970s will walk free from jail within weeks, after successfully appealing the length of his sentence.

The now-retired priest, James Henry Scannell, had served 138 days of his two year sentence when the Victorian Court of Appeal reduced his jail time to 15 months, with 10 months of this suspended.

Scannell was found guilty of a single charge of buggery by a Victorian County Court jury in July, and he continues to deny committing the assault.

The joint ruling by Justices Mark Weinberg, Phillip Priest, Lex Lasry on Tuesday rejected Scannell's bid to have his conviction overturned but did allow a reduction in his prison term.

Scannell's was originally required him to serve at least 12 months before being eligible for parole.

Under the reduced sentence, he will be released within weeks.

Scannell committed the abuse during a sexual education lesson more than 40 years ago.

He was asked by a long-time friend and member of his East Kew parish to discuss puberty with the boy.

Instead, Scannell, wearing only a dressing gown, led the boy into his bedroom, stripped him naked and sexually abused him.

The victim came forward in 2010 when he learnt Scannell was to conduct his aunt's funeral.