The purpose of this paper is to indicate theoretically how much the magnetic field influences the collision frequency for the nuclear fusion reaction. Since the Larmor radius, rL, of a particle in a usual magnetic field strength is large compared to the radius of the cross section, sigma , for the nuclear fusion reaction, the magnetic field does not affect the cross section. However, due to the gyration of the particle during a free path, the relative velocity is affected. First, the flow of charged particles across an area sigma per unit time is obtained, and then the collision frequency in the presence of the magnetic field is determined. It is shown that when 2 pi rL is small compared with the mean free path lambda , the velocity component contribution to the collision is the velocity component due to the motion along the field axis and not that due to the gyration.