The purpose of this study was to evaluate the antimicrobial effect of root canal cements such as AH-26, Sealapex, Canals and Apatite root sealers(Type I, II, III) and to determine the efficacy of their activities. S.mutans(AHT), S.sanguis(NCTC 9811) and B.gingivalis(381) were streaked on blood agar and the PVC tybes filled with root canal cements were applied on. Then the microorganisms were cultured for 48 hours, anaerobically. B.cereus(KCTC 1012) was streaked on nutrient agar, PVC tubes were applied on and were cultured for 48 hours, aerobically. The inhibition zones of root canal cements were measured with vernier caliper. The data statistically analyzed, and the results were as followed. 1. Apatite root sealers(Type I, II, III) showed no inhibition zones. 2. AH-26, Sealapex and Canals had inhibition zones with varying degrees. The inhibition zone of AH-26 was greatest and followed by Canals and Sealapex(P<0.01). 3. As time goes by after mixing the root canal cements, AH-26, Canals and Sealapex showed significantly reducing inhibition zones(P<0.01). 4. There were the least inhibition zones of all the root canal cements on S.mutans and followed in such order as; B.gingivalis, S.sanguis and B.cereus(P<0.01).