SiPM (silicon photomultiplier), which is 2D arrays of avalanche photodiodes operated in Geiger mode, is one of the candidates for photosensors of PET. In this paper, DOI-PET detector, which consists of 2 SiPM of 1.5 mm times 1.5 mm pixel size and 3 cm long LSO crystals, was designed using TCAD and DETECT2000. Because light photon distribution of scintillation crystal becomes different with respect to surface treatments, simulations to improve of DOI information were done with different surface conditions. Side surface treatments of LSO, which have best DOI information, were determined by DETECT2000 simulation. High quantum efficient SiPM was also designed using TCAD. In order to enhance quantum efficiency of SiPM, shallow junction is made by implantation and anti-reflection coating is also optimized.