In photodynamic therapy (PDT) for glioma patients, apoptosis not necrosis is the desirable mode of cell death, as necrotic cell death induces late appearance of obstacles following PDT. We previously demonstrated that increase in both treatment dose of photosensitizer talaporfin sodium (NPe6) and laser fluence (laser energy density) changes the dominant cell death process from apoptosis to necrosis in glioblastoma T98G cells. Here, we investigated the effect of laser irradiance (laser power density), which is another important parameter of PDT, on PDT-induced cell death modalities in cultured T98G cells. When fluence was fixed at 10 J/cm², NPe6 dose-dependently reduced the cell viability, regardless of irradiance (11, 22, 33, and 44 mW/cm²). Morphological observations and biochemical analysis (measurement of caspase-3 activity, staining of cell surface-exposed phosphatidylserine, and staining of propidium iodide) further confirmed that increase in dose of NPe6 changed the dominant cell death process from apoptosis to necrosis, regardless of irradiance. We also noted no influence of irradiance level on the leakage of lactate dehydrogenase from T98G cells following PDT treatment. Taken together, our present and previous findings suggest that dose of NPe6 and laser fluence but not laser irradiance are important parameters to consider in PDT using NPe6 in T98G cells.