Dissipation processes responsible for fast magnetic reconnection in collisionless plasma are investigated using 3D electromagnetic particle-in-cell simulations. The present study compares two simulation runs; one with small system size in the current density direction, and the other with larger system size. In the case with small system size, the reconnection processes are almost the same as those in 2D reconnection, while in the other case the drift kink mode evolves along the current density and deforms the current sheet structure drastically. Although fast reconnection is achieved in both the cases, it is found that the dissipation mechanism is very different between them. In the case without kink mode, the electrons transit the electron diffusion region without thermalization, so that the magnetic dissipation is supported by the inertia resistivity alone. On the other hand, in the kinked current sheet, the electrons are not only accelerated in bulk, but they are also partly scattered and thermalized by the kink mode, which results in the anomalous resistivity in addition to the inertia resistivity. It is discussed that in 3D reconnection the thickness of the electron current sheet becomes larger than the local electron inertia length.
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