Boron Neutron Capture Therapy (BNCT) is an approach of cancer treatment using high reactivity of boron with neutron. In BNCT, image-based monitoring of the reactivity in target region is important for the safety of patient and the evaluation of efficacy in treatment. In this study, we designed and evaluated a system of GAGG based prompt gamma ray imaging (PGI) camera with parallel hole collimator using Monte Carlo (MC) simulation. GATE (v7.0) packages have been used in the MC simulation. Various parameters for parallel hole collimator design were considered. In the MC simulation, a spherical water phantom was placed at the center of field of view (FOV) of gamma camera and a spherical boron phantom was positioned at the center of the water phantom. The diameter of water phantom was changed to evaluate the effect of scatter and annihilation photons in PGI. As the neutron energy increased, the range of the neutron in the water increased. As the amount of scattering material increased, image quality was decreased due to the increased amount of scatter and annihilation gamma ray of 511 keV components. The septal penetration ratio of 3.5 %, which is generally considered suitable to nuclear medicine imaging, was not acceptable in PGI for differentiating target. For PGI in BNCT, higher value of septal penetration fraction should be considered in collimator design. Optimized design of collimator and scatter correction would be necessary for the better quality of PGI of BNCT.