The nonlinear development of magnetic reconnection in an isolated current‐sheet system is studied numerically. The reconnection processes develop from an initial small disturbance under different sets of typical boundary conditions. The following distinct macroscopic characteristics are shown. In a situation where plasma can freely flow out of or into the system, the fast reconnection mechanism by which the stored magnetic energy can explosively be released is set up. The magnitude of the initial disturbance influences the onset time of the fast reconnection but has little effect on the established configuration of fast reconnection. Also, even in the presence of impermeable walls placed at the top and bottom boundaries, the fast reconnection is eventually available, so that the fast reconnection process is regarded as a gross instability inherent to the system itself. On the other hand, in the tearing‐type field geometry with impermeable walls, plasma confinement in magnetic islands prevents the fast reconnection from taking place and finally leads to an oscillation on the nonlinear saturation level.