Abstract: By the use of 500 mb northern hemispheric data of four months during the winter season of 1976-77 medium range variations of northern hemispheric mean kinetic energy, eddy kinetic energy spectra and the spectra of kinetic conversion rate are studied.Through the qualitative analysis and power spectral analysis of these time series, some preliminary results are obtained. Mean energy, total eddy energy and total value of conversion rate all show a quasi-two-week oscillation. But, there are phase differences among them. The total eddy energy leads the total conversion rate by about one day, and leads the mean energy by about 3 days. The values of eddy energy and conversion rate of wave number three all show a quasi-two-week variation. The eddy energy of wave number three is not only in approximate phase with the total eddy energy, but also the most significant contribution among the waves. The wave number 3 also gives the most significant contribution to the total conversion rate. The contributions of conversion rate to the intensification and decay of mean energy and eddy energy for various wave numbers are investigated. It is found that the rates of change of mean energy throughout its intensification are almost balanced by the conversion rates. It suggests that barotropic process plays a more important role in the intensifying stage of mean energy. As far as individual wave numbers are concerned, the decay of kinetic energies for k=3-6 is, say the least, partly due to losing energy through barotropic conversion process.Finally,the relationship between the medium range variations of mean and eddy kinetic energies and their intensifiction and decay is discussed.