Using NCEP2 reanalysis data from 1979 to 2004, the daily 500-hPa geopotential height in the Northern Hemisphere (20°N－90°N) has been expanded into double Fourier series and the signals in the frequency domain have been analyzed for different spatial scales. For the daily series in the entire year, when both the zonal wave number k and the meridional wave number l equal zero, the signals series are characterized by a significant low-frequency variation (10－30 days). When the meridional wave number l equals zero, the signals in the frequency domains vary with the zonal wave number k significantly. The coefficient series consist of a low-frequency variation at larger spatial scales and a high-frequency variation (4－8 days) at smaller spatial scales. When the zonal wave number k equals zero, the signals in the frequency domains vary little with the meridional wave number l. The coefficient series are characterized by a low-frequency variation. When both k and l are not equal to zero, the spatial scales of low-frequency variations mainly concentrate on l=1, corresponding to the dipole structure of low-frequency flows in the mid-high latitudes. The results for daily series during the winter or summer half year are similar to those during the entire year. The diagnosis results can be explained, to a certain extent, by the dispersion relation for the Rossby wave.