Background: MTA is composed of fine hydrophilic particles; its main components are tricalcium silicate, tricalcium aluminate, tricalcium oxide, silicate oxide, and other mineral oxides. The present study was conducted to compare the effect of Portland cement accelerators on the pH and antimicrobial activity of white Portland cement and white ProRoot mineral trioxide aggregate. Materials & Methods: 60plastic tubes were filled with the test material and immersed in deionized water. The pH of the water was measured at 0, 10, 30, 60 minutes and 24 hours using pH meter. The antimicrobial activity of the cement was evaluated using agar-diffusion test. The plates containing BHI agar were inoculated with microorganisms Pseudomonas aeruginosa (ATCC 27853), Streptococcus sanguis (ATCC 10556), Enterococcus faecalis (ATCC 29212) and Candida albicans (ATCC 5314) (HiMedia). Results: Themean pH in group I was 11.5, in group II was 11.3, in group III was 11.1, in group IV was 10.6, in group V was 10.5, in group VI was 11.6, in group VII was 11.9, in group VIII was 12.5, in group IX was 10.6 and in group X was 10.4. The difference was significant (P< 0.05).There was significant difference in antimicrobial activity of P. aeruginosa, E. faecalis, S. sanguis and C. albicans in all groups (P< 0.05). Conclusion: The use of 15% calcium chloride revealed enhanced antimicrobial activity of WPC and WMTA due to its high pH values.