A power transmission system and an attenuation mechanism are disclosed. A power transmission system includes an electric motor, a drive wheel, a transmission shaft and an attenuation mechanism. The transmission shaft transmits a torque between the electric motor and the drive wheel. The attenuation mechanism is attached to the transmission shaft. The attenuation mechanism includes a support member and an inertia member. The support member is attached to the transmission shaft. The inertia member is disposed to be rotatable relative to the transmission shaft. The inertia member is engaged with the support member by friction.