The critical voltage of hard coat range of Al alloys (K 186, 43S) at various electrolytes and the effects of voltage, temperature of electrolytes and concentration of sulfric acid were investigated, m the hope that to find the adequate voltage. Two kind of A1 alloys were anodized in three typical electrolytes and micro vickers hardness of the films were measured. With respect to the relationship between the hardness of the film and the voltage The result of this experiment are as fallows. 1. The critical voltage of hard coat rangs for the electrolytes $H_2SO_4;10%,;H_2SO_4;10%;+;H_2C_2O_4;2H_2O;10g/l,;H_2SO_4;10%;+;NaHSO_4;5%$ was 20V 22V 24V respectively. 2. $H_2SO_4;10%;+;NaHSO_4$ 5% electrolyte was most adventageous among the three electrolyte in the respect of hardness. 3. The effect of concentration of sulphuric acid was not appeared.