A trip mechanism for a circuit breaker, whereby the circuit breaker is provided with a stationary contactor, a movable contactor movable to an open circuit position separated from the stationary contactor and a closed circuit position contacted with the stationary contactor, and a switching mechanism configured to drive the movable contactor to the open circuit position or the closed circuit position, and whereby the trip mechanism comprises: a coil configured to generate a magnetic absorption force due to a fault current on a circuit; a stationary core fixed to a lower portion of the coil within the coil; a movable core movable to the stationary core within the coil by the magnetic absorption force generated from the coil; a bias spring located atop a winding of the coil and the bias spring applies elastic force to the movable core to keep a certain distance between the stationary core and the movable core; a bimetal assembly (217, 218, 218a) electrically connected to a circuit between a power source and an electrical load, and thermally displaced upon occurrence of an overcurrent on the circuit, comprising: a heater electrically connected to the circuit between the power source and the load and configured to be heated up by the overcurrent on the circuit; a bimetal having the heater wrapped thereon and bent by the heating of the heater; and a protruding member disposed at an upper portion of the bimetal to protrude from the bimetal toward the switching mechanism, wherein the protruding member pushes the switching mechanism to trigger the trip operation of the switching mechanism as the bimetal is bent; and a trip lever rotatably connected to the movable core so as to be pulled by the movable core when the movable core is downwardly moved towards the stationary core, and configured to trigger the switching mechanism such that the movable contactor is moved to the open circuit position; characterized in that the movable core comprises a pair of circular plate portions outwardly protruding from an upper end thereof in a radial direction and a neck portion formed between the pair of circular plate portions; the trip lever comprises hinge portions at both ends thereof and a channel portion engaged with the neck portion of the movable core; the bias spring, the movable core and the trip lever are vertically stacked along a common axis vertically extending from a lower supporting plate; the bottom end of the heater is connected to the lower supporting plate; and the protruding member protruding towards the switching mechanism is arranged above the trip lever. a coil configured to generate a magnetic absorption force due to a fault current on a circuit; a stationary core fixed to a lower portion of the coil within the coil; a movable core movable to the stationary core within the coil by the magnetic absorption force generated from the coil; a bias spring located atop a winding of the coil and the bias spring applies elastic force to the movable core to keep a certain distance between the stationary core and the movable core; a bimetal assembly (217, 218, 218a) electrically connected to a circuit between a power source and an electrical load, and thermally displaced upon occurrence of an overcurrent on the circuit, comprising: a heater electrically connected to the circuit between the power source and the load and configured to be heated up by the overcurrent on the circuit; a bimetal having the heater wrapped thereon and bent by the heating of the heater; and a protruding member disposed at an upper portion of the bimetal to protrude from the bimetal toward the switching mechanism, wherein the protruding member pushes the switching mechanism to trigger the trip operation of the switching mechanism as the bimetal is bent; and a heater electrically connected to the circuit between the power source and the load and configured to be heated up by the overcurrent on the circuit; a bimetal having the heater wrapped thereon and bent by the heating of the heater; and a protruding member disposed at an upper portion of the bimetal to protrude from the bimetal toward the switching mechanism, wherein the protruding member pushes the switching mechanism to trigger the trip operation of the switching mechanism as the bimetal is bent; and a trip lever rotatably connected to the movable core so as to be pulled by the movable core when the movable core is downwardly moved towards the stationary core, and configured to trigger the switching mechanism such that the movable contactor is moved to the open circuit position; characterized in that the movable core comprises a pair of circular plate portions outwardly protruding from an upper end thereof in a radial direction and a neck portion formed between the pair of circular plate portions; the trip lever comprises hinge portions at both ends thereof and a channel portion engaged with the neck portion of the movable core; the bias spring, the movable core and the trip lever are vertically stacked along a common axis vertically extending from a lower supporting plate; the bottom end of the heater is connected to the lower supporting plate; and the protruding member protruding towards the switching mechanism is arranged above the trip lever.