When we conduct various kinds of experiments with nuclear reactors and interpret the obtained measurement results qualitatively and quantitatively, knowledge of the theory of nuclear reactor physics is quite helpful and it enables us to better understand the experiments. Phenomena occurring in nuclear reactors are generally based on the interections between neutrons and nuclei, so the fundamental knowledge on this interaction are described and several important physical quantities are introduced. Then, the fission chain reactions are interpreted by a classical theory, the six-factor formula. Based on the fundamental quantities in the nuclear reactor physics such as the neutron flux and the reaction rate, the neutron diffusion equation is derived, and the neutron non-leakage probabilities are quantified based on this diffusion theory. Furthermore, the basics of the nuclear reactor kinetics theory are discussed using the point kinetics equation.