A man is being airlifted to Royal Perth Hospital after the motorcycle he was riding hit a tree east of Waroona.
The man came off his bike near the intersection of Nanga Brook Road and Nanga Road about 10.50am.
A WA Police spokeswoman said the man received serious injuries.
The RAC Rescue helicopter was sent to the scene.
Meanwhile, another man is in a critical but stable condition after a separate motorcycle crash in Warnbro last night.
The man was travelling along The Avenue when he came off his bike about 11.10pm.
He was initially taken to Rockingham Hospital with serious injuries but later transferred to RPH.
A third person was in a stable condition at RPH following a two-car collision in Jandakot this morning.
The crash is understood to have happened about 5.35am at or near the south-bound Berrigan Drive off-ramp on the Kwinana Freeway.
Police believe alcohol may have been a factor in the crash.