We investigated the correlation between the therapeutic effect by early irradiation Photodynamic Therapy (PDT) and vascular response. The early irradiation PDT has been proposed by our group. This PDT protocol is that pulse laser irradiates to tumors 1 h after intravenous injection of water-soluble photosensitizer. The intact layer appeared over the well treated layer, when the early irradiation PDT was performed at rat prostate subcutaneous tumors with high intensity pulse laser (over 1. MW/cm2 in peak intensity) and Talaporfin sodium. In order to clarify the phenomenon mechanism, we monitored blood volume, surface temperature, photosensitizer amount, and oxygen saturation during the PDT. The rat prostate subcutaneous tumor was irradiated with excimer dye laser light at 1 h after the intravenous injection. The photosensitizer dose was 2.0 mg/kg, and the pulse energy density was 2.5 mJ/cm2 (low intensity) or 10 mJ/cm2 (high intensity). Under the low intensity pulsed PDT, the fluorescence amount was decreasing gently during the irradiation, and the blood volume and oxygen saturation started decreasing just after the irradiation. Under the high intensity pulsed PDT, the fluorescence amount was decreased rapidly for 20 s after the irradiation started. The blood volume and oxygen saturation were tempo-rally decreased during the irradiation, and recovered at 48 hrs after the irradiation. According to these results, under the low intensity pulsed PDT, the blood vessel located near the surface started closing just after the irradiation. On the other hand, under the high intensity pulsed PDT the blood vessel was closing for 20 s after the irradiation started, moreover, the blood (low recovered at 48 hrs after the irradiation. We concluded that the vascular response depended on the pulse energy density, and then the therapeutic effect was attributed to the difference of the vascular response. In other words, the surface intact layer could be considered to be induced the temporal drug and oxygen depletion effect associated with the temporal vascular shutdown.