Bacterial resistance against antibiotic treatment is becoming an increasing problem,research is being directed towards other approaches to achieve bacterial killing,including photodynamic therapy.In this study the photodynamic inactivation of Gram-negative bacteria Escherichia coli and Gram-positive bacteria Bacillus cereus by a second generation photosensitizer-hematoporyrin monomethyl ether (HMME) were investigated through the reduction of colony unit and bacteria inactivation rate Results showed that 90% of Bacillus cereus were photoinactivated and the growth was delayed by illumination with 30 min visible light in the presence of 5 μg/mL HMME.