The photodynamic therapy (PDT) is a combination of using a photosensitizer agent, light and oxygen that can cause oxidative cellular damage. This technique is applied in several cases, including microbial control. Various porphyrin and metalloporphyrin compounds are used as photosensitizer agents. In this study, the effect of a tetra-cationic porphyrin and its zinc compound was investigated on Bacillus subtilis under irradiation with a tungsten lamp as a visible light and the illumination time varied from 0 min to 30 min. Increasing the illumination time reduced the colony number, through the disorganisation of the bacterial cell wall.