An independent arbitrator has ordered the D.C. police department to reinstate a homicide investigator who won a protracted unfair labor practice complaint against the department last August for harassing him because of his union activities.

The 24-page decision found that the department's refusal to reinstate Detective William Corboy to the homicide branch was "due to discrimination and reprisal against him."

Last August, the D.C. Public Employees Relations Board upheld a hearing examiner's ruling that Corboy and a former fellow homicide investigator, Thomas Kilcullen, were harassed and improperly punished for their union involvement.

That ruling was based on a labor grievance that Corboy filed after being ousted from the homicide branch in November 1983. At the time, Corboy, then an officer, had been on a "temporary detail" to the homicide branch for more than three years, performing the duties of a detective without having been promoted to the rank or receiving a detective's wages.

After Corboy filed his complaint, he and Kilcullen were broken up as an investigative team and their days off were changed. Corboy was eventually removed from the homicide branch and was transferred to the 4th District, where he severely injured his leg in a motorcycle accident that left his future as a police officer in doubt.

The employes' board hearing examiner subsequently ruled that Corboy and Kilcullen had been subjected to arbitrary threats and reprisals by police officials, and ordered the department to "cease and desist" from discriminating against the pair. Corboy was subsequently promoted to detective.

After the August ruling, D.C. Police Chief Maurice T. Turner Jr. said Corboy would be allowed to return to the homicide branch as soon as he could pass a physical examination. But after his leg healed, Corboy, who has returned to work in the 4th District, applied for a job in homicide but was not selected.

In the latest ruling, the arbitrator found that all others on temporary details in the Criminal Investigation Division had been granted permanent jobs. The arbitrator said there was an upcoming vacancy in the homicide branch at the time Corboy applied and found no valid reason why Corboy was not reinstated.

The ruling stated that the arbitrator will retain jurisdiction over the case for 60 days "for the sole purpose of resolving any disputes concerning the implementation of this award."

Turner had no comment on the arbitrator's ruling, a police spokesman said yesterday. Officials said an appeal was unlikely.