An adjustable thermal trip mechanism for a circuit breaker is provided which can improve the reliability of over-current tripping by minimizing an influence upon thermal tripping even if an assembly error such as skewing or twisting occurs during assembly of bimetallic strips. The adjustable thermal trip mechanism for the circuit breaker comprises: a crossbar that is rotatable and has at least one power receiving portion for receiving rotary power; a bimetallic strip that can bend towards the power receiving portion when an over current occurs on the circuit; and an adjustment screw installed to face the power receiving portion, wherein the power receiving portion comprises a plurality of planar portions which are at different distances from the adjustment screw.