The mechanism of leakage current reduction in phosphorus‐doped polysilicon oxide by high temperature annealing prior to low temperature oxidation was investigated by bias polarity dependence of oxide leakage current, polysilicon grain size measurement, and polysilicon surface observation. Increased phosphorus concentration in polysilicon makes the grains larger and smoothens the surface roughness; consequently, the highest electric field is obtained at the phosphorus concentration of about . Even at the optimum concentration, leakage current due to field enhancement at the interface between polysilicon oxide and polysilicon is still high. Leakage current can be reduced remarkably by high temperature annealing prior to oxidation, especially at the optimum concentration. The annealing effect is explained by interface flatness improvement due to grain growth during annealing and increase of phosphorus atoms in grains. Phosphorus atom migration from the grain boundaries inside the grains is considered to enhance the grain growth.