Prince George's County Police Chief John W. Rhoads, 43, was granted a medical disability retirement yesterday that will permit him to leave office June 29 on a pension that could total $29,700 a year and may be tax-free.

Rhoads received retirement approval from the county Disability Review Board on the advice of a medical panel that found he was permanently disabled by a job-related back injury - a slipped disk.

That finding gives Rhoads a pension at 70 percent of his salary, rather than the usual 50 percent. Job-related disability pensions have hitherto been tax-exempt, but the tax law is being reviewed by the Internal Revenue Service and Rhoads could be required to pay taxes on his pension starting next year.

The chief plans to retire before July 1 because of a county law that will go into effect that day making officers who have been on the police force longer than 20 years ineligible for job-related disability retirement benefits. Rhoads has been on the county force for 22 years. Under current law an officer can work for 25 years before becoming ineligible for such benefits.

Rhoads, who has been chief for four years, injured his back in 1971 when he was in an auto accident and reinjured it in 1974 at the scene of an explosion when a truck backed into him.

The injury was aggravated early this year when he slipped on ice and again in early April when he slipped on wet ground after addressing officers at a Fraternal Order of Police rally in the county. Rhoads has been at work only sporadically since April.

Despite controversy during Rhoads' years as chief involving relations between the predominantly white police force and the county's growing black community, he was widely praised yesterday after the review board announced its decision.

County Executive Lawrence J. Hogan called him "a credit to law enforcement in general and to Prince George's Count in particular."

Laney Hester, president of the county police union, whose organization once voted "no confidence" in Rhoads, said he hopes the new chief [not yet chosen] will be like Rhoads, "I enjoyed working with him and found him to be a real gentleman," he said.

County Council member Sue V. Mills objected to the retirement law and said, "He shouldn't have been considered disabled until after he'd had an operation. I had the same back problem and I had an operation. I'm fine now."

Rhoads has said that if his back injury were cured he would consider going back to police work because "it's in my blood." If he did take another job, the disability board would review his case and could reduce his pension.