The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was constructed and mounted in a 3-T MRI. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geiger-mode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. GAPD output charge signals were transferred to preamplifiers using flat cable of 3 m long, and then sent to position decoder circuit (PDC) identifying digital address and generating an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. LYSO and GAPD were located inside MR bore and all electronics including preamplifiers were positioned outside MR bore to minimize signal interference between PET and MR. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI using the MR compatible PET system. The rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. Activity distribution patterns between white and gray matter in Hoffman brain phantom were well imaged. No degradation of image quality of the hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences was observed. These results demonstrate that simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.