It is well known that Stratocumulus(Sc) frequently occurs over subtropical oceans west of the continents.However,Sc is also widespread over northern Pacific Ocean during summer.Two regions with typical summertime Sc over northern Pacific Ocean are selected: one is located off the west coast of Californian(NEP),while the other is located over northwestern Pacific(NWP).The datasets of ISCCP D2 Sc cloud amount,ERBE radiation data,NCEP/NCAR reanalysis data and NOAAOI-SST are used in this paper.Different from the previous criterion,moist stability criterion about the Sc cloud is suggested to analyze the relationship between Sc cloud amount and stability,because the moist stability criterion has clearer physical meanings.It is shown that the two regions,NEP and NWP,both are with positive moist static stability,which is favorable of the formation and persistence of Sc.There are distinct differences of air-sea temperature difference,latent heat and sensible heat flux between NEP and NWP.The analysis of lead/lag correlations shows that SST lags Sc three months at NEP,while SST leads Sc one month at NWP.The former may be attributed to the huge heat inertia of ocean,and the later may be responsible for the favorable condition due to the negative air-sea difference and the advection of warm air into that region.