The dynamical development of collisionless reconnection and the consequent energy-conversion process in the presence of an external driving flow are investigated by means of a full particle simulation. Magnetic reconnection develops in two steps in accordance with the formation of ion and electron current layers. In the early phase magnetic reconnection is controlled by an ion kinetic effect, while an electron kinetic effect becomes dominant in the late phase. There exist two mechanisms associated with the particle kinetic effects, that break the frozen-in condition of magnetic field and lead to magnetic reconnection in a collisionless plasma, namely a particle inertia effect and a particle thermal orbit effect. It is found that the dominant triggering mechanism in the late phase changes from an electron thermal orbit effect to an electron inertia effect as the longitudinal magnetic field increases. Electron acceleration and heating take place in the reconnection area under the influence of the reconnection electric field, while the energy conversion takes place from electrons to ions through the action of an electrostatic field excited downstream. As a result, the average ion temperature becomes about 1.5 times the average electron temperature.