Mesoscale eddy effects on the wintertime verti-cal mixing in the formation region of the North Pacific Sub-tropical Mode Water (NPSTMW) are studied using hydro-graphic data from Argo profiling floats deployed in the Ku-roshio recirculation region in February and March of 2001. Anticyclonic (warm) eddy enhances the wintertime vertical mixing and results in the deep mixed layer and the deep thermocline. Consequently, a large volumetric water mass with low potential vorticity corresponding to the prototype of NPSTMW tends to be formed. By contrast, cyclonic (cold) eddy is unfavorable for the vertical mixing process and halts the deepening of the mixed layer and thus the formation of mode water. Further analysis shows that cyclonic eddies prevail in the late 1990s in the formation region of NPSTMW, which lead to significant suppression of the wintertime ver-tical mixing (96-98) and thus are unfavorable for the for-mation of NPSTMW; while the situation is completely re-versed in the early 1990s (93-95).