Arson, the leading cause of fires in Prince George's County as recently as 1978, has declined 27 percent in the last four years, county officials said yesterday.

Fire Chief Marion (Jim) Estepp, at a press conference in Upper Marlboro, credited a round-the-clock arson investigation unit, greater coordination with law enforcement agencies and increased public awareness for the decline in intentional fires.

There were 965 arson fires in 1979, causing $10.7 million in damage, compared with 736 fires and $9 million in damage in the fiscal year that ended July 1. Fire department investigations led to arrests in 36 percent of arson cases last year, compared with 9 percent nationally, Estepp said. Almost 95 percent of the arrests resulted in convictions.

"The word on the street is that if you commit arson in Prince George's County you will go to jail," Estepp said.

Arson was the second leading cause of fire in the county last year, behind cooking related accidents.

In Montgomery County, where arson continues to rise, according to fire department spokesman Raymond Mullhall, County Executive Charles Gilchrist created an arson task force to deal with the problem.

More than 60 percent of arsonists in Prince George's are between 12 and 14, acting out anti-social behavior or seeking attention, according to Ward Caddington, chief of the investigative unit. Arson for insurance fraud accounted for only 15 percent of the fires. As a result, the fire department has produced a professional looking slide show: Arson: Countdown to Tragedy, featuring grim photos and interviews with fire victims aimed at adolescents.

The film will be shown to the 58,000 county secondary school students this year.

Nationally, Estepp said, the incidence of arson has increased 300 percent over the last decade and continues to climb.