The 29-year-old Rebels bikie from Merriwa was riding with a group of 90 other people towards Waroona about 3.15pm.
Police said he appeared to have lost control of his Harley Davidson motorcycle on Peppermint Grove Road, about 2km from Forrest Highway, in Lake Clifton.
The motorcycle struck a ditch and became airborne.
He died as a result of the injuries he received in the crash.
People who saw the crash or the bike beforehand are asked to call Crime Stoppers on 1800 333 000.