For the high quality Si Photodetector, the high resistivity epitaxial wafer using the low resistivity substrate were studied. The buffer layer was introduced in the interface, and it was very effective on the crystal quality of the epitaxial layer. Recombination lifetime in the epitaxial layer became very uniform and long even in the interface region which was confirmed by measuring the lifetime depth profiles. Then Si PIN Photodiode was fabricated on the above high quality epitaxial wafer and its optoelectric characteristics was evaluated.