A failure diagnosis method for a brake system of a vehicle determines whether the failure occurs in a regenerative braking device, a front-wheel friction braking device or a rear-wheel friction braking device. The method may include comparing a driver's required acceleration/deceleration with a current acceleration/deceleration of the vehicle, determining that a failure has occurred in the brake system of the vehicle when the driver's required acceleration/deceleration is different from the current acceleration/deceleration of the vehicle and comparing a driver's required braking torque with a regenerative braking maximum torque, and determining that the failure does not occur in the brake system of the vehicle when the driver's required acceleration/deceleration is substantially equal to the current acceleration/deceleration of the vehicle.