Photodynamic therapy (PDT) is a safe and alternative antimicrobial treatment that consists of a chemical agent, called photosensitizer, which can be activated by light of an appropriate wavelength to produce reactive oxygen species (ROS). PDT can be used for photoinactivation of bacteria in an attempt to overcome the problem of bacterial multidrug resistance. In particular, it is an effective antimicrobial treatment against infected wounds that have antibiotic resistance and wound infections would otherwise lead to mortality and morbidity. The main purpose of this study was to demonstrate the importance of PDT dosimetry (light dose and concentration of photosensitizer). If the dosimetry of PDT was not optimized properly, photoinactivation of bacteria cannot be achieved and even worse biostimulation on pathogens could be observed. This study investigated whether there is a biostimulative effect due to free oxygen radicals of PDT when light dose and photosensitizer concentration are too low. In this study, the biostimulative effect on P. aeruginosa strain was observed instead of the PDT effect, when 84 J/cm2 of energy dose (809-nm diode laser) was applied with 20, 50, 100 and 150 μg/ml of ICG concentrations. The killing effect of PDT was observed with higher ICG concentrations, such as 200, 250 μg/ml of ICG. However the killing effect was not enough to destroy pathogen efficiently with these high concentrations of ICG.