A time series of the core layer temperature (CLT) of the North Pacific subtropical mode water for the 40-year period from 1957 to 1996 is constructed using all available temperature profiles archived in World Ocean Database 1998. CLT is defined as the temperature of the layer having the minimum vertical temperature gradient at a single profile, with temperature ranging from 15 to 19°C. Annual mean CLT is calculated using all CLT data taken from May to December for four 5° × 10° (lat. × Ion.) regions from 30° to 35°N and from 140°E to 180°. The CLT time series shows decadal to interdecadal variation, as well as interannual variation. This time series correlates very well with that of wintertime (February and March) sea surface temperature in the same region. Relationships are investigated among several atmospheric circulation indices, the Kuroshio transport, and CLT. As a result, it is suggested that CLT might be crucially influenced by the Kuroshio transport in a time frame longer than ten years, and by the wintertime East Asian Monsoon in shorter time frames.