The antibacterial activity of citrus peel extracts was measured using disc diffusion assay followed by broth dilution assay in order to determine MIC. Various degrees of inhibition against pathogenic bacterial strains were shown and the results were given in Table 2. It was evident that solvent type affects antibacterial activity. Ethyl acetate showed antibacterial effect against all strains while ethanol showed no effect against E. coli. KEa inhibited E. coli showing the greatest inhibition zone of 22.3±0.2 mm with the lowest MIC/MBC determined at 0.4 mg/ml, while PEa showed no inhibition zone against E. coli. Moreover, KEa exhibited no significant difference compared with the positive control, chloramphenicol showing inhibition zone of 25.0±0.3 mm against E. coli (data not shown). S. typhimurium and S. aureus were also found to be susceptible to KEa, showing inhibition zone of 19.0±0.0 and 18.0±0.1 mm, with MIC/MBC of 0.8 and 6.3 mg/ml, respectively. The antibacterial activity of citrus peel extracts was in the following order: Kaffir lime > Lime > Pomelo. Base on KEa data, the susceptibility of the test bacteria was in the following order: E. coli > S. typhimurium > S. aureus > B. cereus. These results were consistence with the previous studies on antibacterial effects of other plant extracts generally showing higher ability to inhibit Gram positive rather than Gram negative bacteria (Oke et al., 2009; Melendez & Capriles, 2006). Therefore, KEa is an expectable candidate as a natural antibacterial agent effective against broad spectrum of both Gram positive and Gram negative bacteria. Furthermore, KEa obtained in this study exhibited more effective against Gram negative bacteria than Kaffir lime essential oil reported in previous studies having MIC against E. coli and S. typhimurium at > 2.5 mg/ml (Chanthaphon, et al., 2008).