Antimicrobial activities of green tea extracts used for coarse tea were investigated by disc diffusion method using eight different bacteria. Among the green tea extracts, the 70% ethanol extract demonstrated the strongest antimicrobial activities against Vibrio parahemolyticus (V. parahemolyticus) and Staphylococcus aureus (S. aureus) and thus was further fractionated. Among these fractions, the ethyl acetate fraction showed the strongest antimicrobial activities against V. parahemolyticus, S. aureus, Bacillus subtilis (B. subtilis), and Streptococcus mutans (S. mutans). These activities exceeded that of all extracts and fractions tested in this study. Interestingly, although green tea extracts showed significant antimicrobial activity against Micrococcus luteus (M. luteus), once fractionated, the ethyl acetate fraction did not show any antimicrobial activity against M. luteus. MICs of the ethyl acetate fraction were $5;;{mu}L$/disc against B. subtilis and $3;{mu}L$/disc against S. aureus, S. mutans and V. parahaemolyticus. 90% inhibition of B. subtilis was observed with 0.05% ethyl acetate fraction but S. mutans needed over 0.1% ethyl acetate fraction to exhibit the same inhibition as B. subtilis. Antimicrobial activities of ethyl acetate fractions were reduced around 10% by thermal treatment at $121^{circ}C$ for 20 min. All the results suggest that the 70% ethanol extract as well as the ethyl acetate fraction from green tea used for coarse tea could be further developed into a natural antimicrobial agent.