An apparatus of modular trip mechanism and auxiliary mechanism for a circuit breaker comprises an auxiliary mechanism module including a first micro switch to output an electrical signal indicating an ON/OFF position of the circuit breaker, a first shaft contact lever mechanism to operate the first micro switch by contacting the switching shaft or receiving an artificial pressing force, a second micro switch to output an electrical signal indicating whether a trip operation of the circuit breaker has been performed, and a second lever to operate the second micro switch by contacting the switching shaft or receiving an artificial pressing force; and a trip mechanism module including an electromagnetic trip device to operate a trip bar to trigger the circuit breaker to a trip position in response to a trip control signal from an overcurrent relay or a test trip control signal from a test signal generating source.