An assembling mechanism adapted to assemble a lens module to an optical engine base of a projector includes a fixed base, a fixed bayonet, a movable bayonet, a fixed member, and a movable member. The fixed base is fixed to the optical engine base and includes a restricting cavity. The fixed bayonet is fixed to the fixed base and extends to an open end of the restricting cavity. The movable bayonet is fixed to the lens module. The fixed member is fixed to the optical engine base. The movable member is slidably connected to the lens module. Displacement and rotation of the movable bayonet about the axis relative to the fixed base are restricted when the movable bayonet is moved into the restricting cavity through the open end and rotated about an axis relatively to the fixed base, and the movable member is slid into engagement with the fixed member.