This paper compares ad-valorem and specic taxes in terms of welfare when consumers have both a quality and a quantity choice under perfect competition. In the setting, while ad-valorem tax causes only income effect, specic tax because not only income effect but also substitution effect. This implies that if substitution effect exists, specic tax generates welfare losses. Using a constant elasticity of substitution (CES) utility function and a linear price function, we show that ad-valorem tax is superior to specic tax except for Leontief preference under which the two forms of commodity taxes are equivalent in the sense of revenue.