We studied the kinetic process of a folding/unfolding transition of a long semiflexible polymer chain by Brownian dynamics simulation. A semiflexible chain with an elongated coil conformation is folded into a compact and ordered structure, such as a toroid or rod, through the processes of nucleation and growth. This transition is characterized as a disorder-order transition. The folded morphology is not necessarily the global minimum of free energy, which demonstrates the crucial effect of kinetics on the semiflexible chain folding. The growth process of toroid formation is characterized by a constant growth rate reflecting the dimensionality of a chain, which corresponds well with experimental observations of DNA folding. It is also shown that the path of the unfolding process is much different from that of folding.