A balancer of a washing machine including a balancer housing mounted to a front surface of a drum and having an annular channel therein, a plurality of masses movably disposed along the channel, at least one magnet coupled to the balancer housing so as to restrict the masses from moving along the channel when a revolution per minute value of the drum is within a specific revolution per minute range, and at least one guide groove which is formed on an inner surface of the balancer housing and guides movement of the plural masses when the drum rotates.