A MR compatible PET insert has been developed in our group employing SiPM and a charge signal transmission method that transmits the charge signal of SiPM to subsequent electronics through 4 m long cables. The previously developed PET/MRI system, however, had several limitations, such as the instability of PET performance due to temperature variations inside PET gantry and the degradation of SNR in the MR image due to the inserted PET. The purpose of this study was to optimize the design of PET/MRI system to maintain a stable temperature inside the PET gantry and to minimize mutual interference between PET and MRI. The PET insert was composed of 9 PET detector blocks that were arranged in a circle around the subject with 390 mm diameter and 60 mm axial FOV. Each PET detector block was composed of a 4 × 8 array of detector modules consisted of 4 × 4 array of SiPM and 4 × 4 array of LYSO. The PET detectors were cooled using a water chiller system to compensate temperature fluctuations caused by eddy currents. PET analog electronics were enclosed in an aluminum box and located 50 cm apart from the MR bore. The FPGA-embedded DAQ and power supply were installed at outside MR room. PET performance was measured inside and outside MRI using spin echo and gradient echo sequences. MRI performance was evaluated with and without the PET insert. Human brain imaging was also acquired to evaluate the performance of simultaneous PET and MR imaging. No significant degradation of the PET performance caused by MR was observed when the PET was operated under various MR imaging sequences. The homogeneities and SNRs of MR images obtained with and without the PET insert installed inside the MR bore were similar and changed by only 1% and 3%, respectively. High quality simultaneous PET and MR images of human brain were successfully acquired. Experimental results indicate that simultaneous PET and MR imaging is feasible without considerable mutual interference between PET and MRI by employing the PET/MRI proposed in this study.