A climatology of cyclogenesis frequency of extratropical cyclones in the Northern Hemisphere and East Asia for the years of 1958 to 2001, and the interdecadal variation and possible causes, were analyzed by applying an improved objective detecting and tracking algorithm to the six hourly sea level pressure fields in the European Center for Medium range Weather Forecasts reanalysis data ERA40. The result shows that: (1) major source regions of extratropical cyclones in the Northern Hemisphere are eastern North America (downstream areas of Rocky Mountains), Northwest Atlantic, Greenland Northern Europe, Mongolia and Japan Northwest Pacific. Atmospheric baroclinicity and positive vorticity are apt to intensify and develop in ocean west bank and leeward slope of steep orography, which makes for the formation of ground cyclones. (2) The cyclogenesis number of annual, spring and winter indicated a decrease trend in the region of 30°-60°N and an increase trend in the region of 60°-90°N, which supports the viewpoint of the northward shift of storm tracks in the Northern Hemisphere. The cyclone number also had a negative and positive correlation with Arctic Oscillation index (AO) in the south and north of 60°N respectively, especially for annual, spring and autumn. (3) The annual cyclone number in East Asia from 1958 to 2001 indicated obvious interdecadal variations. The cyclone number increased from 1960s to mid 1980s but reduced after mid 1980s in the region of 40°-60°N, 80°-140°E. This is because atmospheric baroclinicity weakened in this region, while strengthened in the higher latitude region, resulting in the northward shift of cyclone source region. There is a linear increased trend of cyclone numbers in the lower latitude region of 20°-40°N, 110°-160°E, mainly because the North Pacific storm track, located in 40°-55°N, had a shift trend to lower latitudes.