Disclosure is a locking mechanism for connectors which are engaged with each other, which prevents the connectors from being insufficiently engaged by inertial locking, and allows the connectors to be engaged or disengaged with ease. The locking mechanism for male and female electrical connectors which are engaged with each others, a guide frame mounted on the upper wall of a male connector housing includes a slide piece which is slidably provided between the upper wall and the guide frame, and a locking spring. One end portion of the locking spring is fixedly secured to the guide frame, and the other end portion is positioned above the front end portion of the slide piece when the latter is at the forward position, and in front of the front end portion when the slide piece is at the backward position. The locking section of the male connector housing and the engaging section of the slide piece hold the slide piece in position. The female connector housing has a connector receiving section. When the two connector housings are engaged with each other, the locking spring is brought in slide contact with the protrusion formed at the connector receiving section.