Photophysical properties and singlet oxygen generation efficiencies of oxa- and/or thia-porphyrins have been investigated by time-resolved and steady-state spectroscopic methods. Introduction of sulfur atom into the porphyrin core induces a slight increment in the internal conversion rate and it also largely increases the intersystem crossing rate due to spin-orbit coupling enhancement, resulting the enhanced triplet state and singlet oxygen quantum yields. However., the quantum yields of mono oxygen-substituted porphyrin are reduced due to relatively more enhanced internal conversion. These suggest that sulfur modification in the core improves the photophysical efficiency of photo sensitizers in the application respect of photodynamic therapy (PDT).