A fuel vapor treatment system is mounted on a hybrid vehicle having an internal combustion engine and an electric motor. Even when an internal combustion engine is stopped, a discharge of fuel vapor from a first canister to atmosphere can be detected. When the discharge of the fuel vapor from the first canister is detected, the internal combustion engine is started to perform a purge process. When it is detected that the purge process in the first canister is finished, the internal combustion engine is stopped to terminate the purge process.