The folding/unfolding kinetics of a three-dimensional lattice protein was studied using a simple statistical mechanical model for protein folding that was previously developed. The model considers the specificity of an amino acid sequence and the native structure of a given protein. The characteristic relaxation rate on the free energy surface was calculated starting from a completely unfolded structure (or native structure) that is assumed to associate with a folding rate (or an unfolding rate). To elucidate the roles of individual amino acid residues in protein folding/unfolding kinetics, the kinetic properties for all possible single amino acid substitutions of these proteins were calculated and their responses were examined. This book presents and discusses research results in the kinetics of protein folding/unfolding.