In order to determine the optimum light irradiation condition to treat deep lesions, we studied influence oflight intensity and repetition rate of nanosecond light pulses on photodynamic therapy(PDT) with PAD-S3 1(1 3,1 7-bis[lcarboxypropionyl]carbamoylethyl-3-ethenyl-8-ethoxyiminoethylidene-7-hydroxy-2,7,12,18-tetramethyl porphyrin sodium) to mouse renal carcinoma cell line(Renca) in vitro. The oxygen consumption and photobleaching were measured to explain this influence. We used the short light pulses(2: 670 nm, FWHM: 5 ns) at the peak intensity of 06, 1 .8 and 3.6 MW/cm2, repetition rate of 30 and 5 Hz, and used the total fluence of 40 J/cm2. We obtained over 80% cell growth inhibition rate at 0.6 MW/cm2 and 5 Hz. This irradiation condition was the lowest peak intensity and lowest repetition rate in our study. From the measurement for oxygen consumption in the well(culture medium) by a microelectrode and oxygen diffusion calculation by the finite element model, we predicted that the low repetition rate may supply sufficient oxygen for PDT by diffusion process. The photobleaching detection by fluorescence measurement showed that bleaching occurred more intensively at lower peak intensity i.e., the PDT process advanced in this intensity. With high peak intensity irradiation such that we used, it is supposed that the waste energy which was not absorbed by photosensitizer to suppress effective light dose for the PDT, since the number of photon per pulse was extremely larger than the number of the photosensitizer molecule in the cell. The PDT effect at our optimum irradiation condition (Over 80% cell growth inhibition rate) was higher than that by continuous wave irradiation at the same average power density and energy dose(40%). Therefore, we conclude that the irradiation condition of low peak intensity(O.6MW/cm2) and low repetition rate (5Hz) using the nanosecond pulsed irradiation was suitable for the PDT in cultured cell in vitro with PAD-S3 I.