A Perth police officer has been found not guilty of using excessive force in an arrest in which a man's jaw was broken.

Constable Adam Wiringi stood trial accused of using excessive force when he arrested 24-year-old David Barnard in February 2007.

It was alleged Constable Wiringi picked up Mr Barnard and threw him head first into the ground.

The officer was trying to arrest Mr Barnard because he had refused to stop after being seen riding in Kiara without a helmet.

Constable Wiringi denied any wrongdoing, saying he had fallen while holding Mr Barnard's arm.

Previously, a witness told the District Court in Perth he saw the officer throw the man head first into the ground.

Eric Owen was the owner of the house where the alleged assault took place.

He said he saw Mr Barnard bleeding quite heavily from his nose and mouth after the incident.

Mr Barnard has testified that just before the alleged assault, he heard Constable Wiringi yell, "wait until I get my hands on you."

He also told the court he was then taken to Kiara police station where he was refused medical assistance.

After being released, Mr Barnard went to hospital and eventually had an operation to insert a metal plate in his jaw.

The District Court jury deliberated for only 15 minutes before finding the officer not guilty.

Constable Wiringi was stood aside from operational duties when he was charged but is now expected to be reinstated to full duties.

The WA Police Union's Russell Armstrong is standing behind the officer.

"We will certainly look after this officer; he was acting in the execution of his duty doing the right thing and doing the right thing by the state of Western Australia," he said.

David Barnard's mother, Denise Hallett, says she is disappointed with the verdict and now plans to take the matter to the Corruption and Crime Commission.

She says her son's health has suffered greatly.

"David tried to commit suicide as a direct result of this trial last year which is why it's taken so long to get to court," she said.

"I believe that justice has not been done."