We studied both the fast and slow modulation of mesoscale variability in the western Kuroshio Extension region (along track 119) by analyzing 4-year TOPEX/Poseidon altimetry data. Bulk of the mesoscale variability lies near the axis of the Kuroshio Extension and the western boundary current along the east coast of Japan. It is found that both the intensity and spreading of mesoscale variability in the frequency band has significant, both abrupt and slow, variations. The slow modulation may be mainly caused by low-frequency variation in the background mean flow, while the abrupt change is primarily due to strong nonlinear processes.