Abstract: Recently, the processes of 2-week and 3-week as well as some other shorter periods of 3-4 days are f ound by many authors. The causes of vacillation in the atmosphere are partly due to the cycles of external forcing effects such as interaction of Atmosphere-Ocean, partly due to the internal effects such as resonance of different length scales of waves. And the latter will be possibly dominate for the medium range processes. By use of the formula of energy variation derived in the previous paper by one of authors, the calculation with the ideal data is made in this paper. It is found that the periods vary widely with different initial value or Ki, but generally there are two distinct types of resonance periods, one is longer than 15-day, the longer type, the other is longer than 3-4 days, the shorter. This is comparable with that of observational results, and this implies that the medium range process is closely related to the resonance of waves.