Abstract: Zonal harmonic analysis are carried out for monthly sea-level pressure along four latitude belts (30°N, 30°S, 50°N, 50°S) for period from 1951 to 1960. The characteristics of the caculated ultra-long wave show that: (1) there are two kinds of variations of the general circulation, seasonal and un-seasonal. In the Northern Hemisphere, in which the continent-ocean contrast is large, the seasonal variation is significant in both subtropical and temperate belts. In the Southern Hemisphere it is dominant only in the subtropical belt, while in the temperate belt the unseasonal variation is more evident. (2) There are significant interannual variations of the seaaonal course in both Southern and Northern Hemispheres. The beginning of summer in the Northern Hemisphere and the ending of summer in the Southem Hemisphere are changed obviously from one year to another. But the beginning of northern summer has much greater stability and so does the ending of southorn summer. (3) The interannual variation of the positions of ultra-long waves associated closely with the distribution of continent and ocean. The dominant ultralong waves are more stable in the latitude belt in which the continents occupy a great portion of the area. In the 50°S belt, in which the ocean consist of 90% or more of the area of latitude belt, tire postions of ultra-long waves variate greatly from one year to another, independent of whether they are dominanting or not in this latitude belt.