Temporal and spatial characteristics of interdecadal SST variability of the North Pacific Ocean were investigated and its effect on ENSO was discussed. Interdecadal variability in SST obviously exists in both the tropical and north Pacific Ocean, while it is relatively weak in the equatorial Pacific and SST changes in the north Pacific possess lower frequencies. 1977 was a turning point for interdecadal SST variations (from cold to warm) in the equatorial Pacific as well as for the SST principal mode (EOF1) of the Pacific SST in terms of temporal change. But the north Pacific changed from warm to cold period from 1973, which is attributed mainly to the second mode. Both equatorial and north Pacific Ocean has turned to warm period since 1990. Revealed by the first mode, SST oscillation of the middle latitude North Pacific Ocean and its adjacent area are out of phase, while oscillation of the north Pacific was revealed by the second mode. Significant impact of interdecadal changes of equatorial SST on ENSO strength is shown. It seems that warm decadal background is a prerequisite condition for frequent and strong ENSO events.