The basic features of nonlinear reconnection and magnetic loop development are studied by computer simulations in a wide variety of physical situations. Initiated by a small disturbance in a long current sheet system, magnetic reconnection builds up, and a magnetic loop is formed near the (left) wall boundary. For the spontaneous fast reconnection model, if the reconnection site is far from the wall, the quasi-steady and symmetric fast reconnection mechanism is set up without any significant dependence on plasma parameters. The supersonic reconnection jet between a pair of standing slow shocks collides with the loop and gives rise to a strong fast shock ahead of the loop top. When the reconnection site is very close to the wall, the fast shock is much weaker and soon decays, and the fast reconnection process becomes asymmetric since the waves, reflected from the loop top, notably influence the X point. For the uniform resistivity model, magnetic reconnection proceeds much more slowly with no definite fast shock formation at the loop top. The X neutral point is not stable but collapses into a long diffusion region, and the elongation of the diffusion region becomes more rapid for the smaller resistivity. In this case, the magnetic energy conversion is performed by the Ohmic heating, whereas it is by the large-scale motor (slow shock) effect for the spontaneous fast reconnection model. ¿ 1999 American Geophysical Union