A castor includes a castor body, an operation unit, a locking unit, and two rollers. The operation unit includes a pressing plate and a rotation member. The locking unit includes a locking module, a stop ring, a locating member, and an upper positioning plate. The locking module has a locking member and a braking member. When the driving portion of the rotation member presses the first ramp of the pressing plate, the locating member is locked in the locking hole of the upper positioning plate. When the driving portion presses the second ramp of the pressing plate, the first engaging portion of the locking member engages the first locking portion of the stop ring, while the second engaging portion of the braking member engages the second locking portion of each of the rollers.