Abstract: Data of cosmic rays observed at Beijing Neutron Monitor Station during magnotic quiet days from solar cycle 22 to solar cycle 23 are analysed to show the diurnal and semidurnal variation characteristics by using Fast Fourier Transformation and power spectrum estimation. The days for data analysis are so selected that not only these days be at least in a successive 6 days’ period of very low magntic activities, but also the previous five days with geomagntic index Ap lower than 15. The computational results show that the periodical features of diurnal and senmidiurnal of the cosmic rays disappeared during the period from one solar cycle to the next solar cycle. When solar activity gradually increased from Solar Minimum to Solar Maximum, the diurnal vartation appeared earlier than semidiurnal variation. The solar diurnal variation’s magnatitude is bigger than that of semidiurnal’s. Diurnal variation of cosmic rays during Solar Meximum of solar cycle 22 is not clearly seen, while on the contrary it is very distinct during solar maximum for solar cycle 23.