The invention relates to a locking mechanism of a molded case circuit breaker. The characteristics lie in that the locking mechanism comprises a fixing base (1), a lock (2) is mounted in a lock hole (101) at one side of the fixing base (1) and capable of rotating, a lock fastener (3) is mounted at the other side of the fixing base (1) and connected with the lock (2), the rotation of the lock (2) can drive the lock fastener (3) to rotate, and the lock (2) is provided with a key (4). The whole locking mechanism is installed inside a housing of the circuit breaker in an accessory mode, the installation process is simplified, and the space occupied by the product is reduced.