Abstract: In this paper, the method of space-time spectral analysis proposed by Haya-shi has been used for the grid data of monthly mean sea level pressure from 10°N to 60°N in 1951-1974. The temporal and spatial behaviour of low freque-ncy transient waves as well as their components-traveling eastward, traveling westward and standing waves-has been obtained: The prevailing periods have been found for each harmonic wave at these latitudes, and their behaviour of propagation has been analyzed. Finally, the relation between the South Oscillation and the planetary wave, harmonic wavenumber 1, in Northern Hemisphere has been investigated. It can be seen that the effect of the South Oscillation on the planetary wave is charaterized by the tropical low frequency waves traveling eastward, and that there is an oscillation system at subtroptical latitudes in Northern Hemisphere, which causes the low frequency standing waves with a period of quasi-three years.