Objective This experimental study was performed to investigate the effect of Coptidis rhizoma extract compared with quantity on Staphylococcus aureus that induce keratitis. Methods Minimal inhibitory concentration(MIC) was measured by dropping to $50{mu}l$　according to density Coptidis rhizoma extract(100%, 10%, 1%, 0.1%) compared with quantity(40g, 80g, 160g). Anti-bacterial potency was measured by the size of inhibition zone with change of volume. Results 1. MIC on Staphylococcus aureus in Coptidis rhizoma extract was showed anti-bacterial potency compared with quantity and density in 100% and 10% of all samples(40g, 80g, 160g). 2. MIC on Staphylococcus aureus in Coptidis rhizoma extract(40g, 80g, 160g) was showed anti-bacterial potency compared with quantity all samples($20{mu}l,;30{mu}l,;40{mu}l,;50{mu}l$) in 100% density. 3. MIC on Staphylococcus aureus in Coptidis rhizoma extract(40g, 80g, 160g) was showed anti-bacterial potency compared with quantity all samples ($20{mu}l,;30{mu}l,;40{mu}l,;50{mu}l$) in 100% density. Anti-bacterial potency of 80g Coptidis rhizoma extract decreased compared with 40g. Anti-bacterial potency of 160g Coptidis rhizoma extract decreased compared with 40g in $20{mu}l,;30{mu}l$. Conclusions Coptidis rhizoma extract was showed anti-bacterial potency compare with quantity and density. In herbal drug, antibacterial potency compare with quantity and density must be studied.