District Attorney Seth Williams, who took office in January, said the policy is meant to help unclog Philadelphia's crowded court dockets.
"We can't declare a war on drugs by going after the kid who's smoking a joint on 55th Street," Williams said. "We have to go after the large traffickers."
Williams' office is working on the change with two members of the state Supreme Court, Chief Justice Ronald D. Castille and Justice Seamus P. McCaffery. They hope the new policy, set to take effect later this month, will sweep about 3,000 small marijuana cases a year out of the main court system so that prosecutors and judges have more time for more serious crimes.
Currently, the maximum penalty for possession for personal use is 30 days in jail, but defendants rarely serve time. Under Williams' predecessor, Lynne Abraham, the office prosecuted about 3,000 such cases a year.
Under the new policy, fines for minor drug possession would likely be $200 for first-time offenders and $300 for others. Offenders with a criminal history could still end up with a formal misdemeanor conviction.
Joseph McGettigan, first assistant district attorney, said people could still be given 30 days in jail but that - as before - such a penalty is unlikely.
People charged with possession for personal use would still be arrested, handcuffed, searched, detained and fingerprinted. Their cases would be heard by a summary court that often handles offenses such as public drinking and disorderly conduct.
Williams' top aides are still researching whether they can simply convert small marijuana arrests into summary charges of disorderly conduct. Some of his advisers believe such a shift would require a change in state law or in a city ordinance.
Several dozen cities have enacted "lowest law enforcement priority" ordinances, stipulating that police pursue such pot-possession cases as a last resort.
Seattle approved a ballot question mandating such a change in 2003. In 2005, Denver voters approved an ordinance legalizing possession of less than an ounce of pot. San Francisco passed a similar law in 2006.
In Massachusetts, the state made possession of less than an ounce of marijuana a civil offense with a $100 fine.
Police have been briefed on the planned change, but they don't plan to stop making marijuana arrests.
"We're not going stop locking people up," said Lt. Frank Vanore, a police spokesman. "Whether or not they make it through the charging process, that's up to the D.A."
