A washing machine includes a cabinet, a drum rotatably arranged within the cabinet, an annular recess provided in the drum, and a balancer to offset an unbalanced load caused within the drum during rotation of the drum. The balancer includes a balancer housing mounted to the recess and has an annular channel therein, at least one mass movably disposed in the channel, and at least one ball disposed between the balancer housing and the mass to generate rolling motion.