Abstract: A Compton camera consisted of two layers of 6×6 position sensitive cerium-doped gadolinium aluminum gallium garnet (GAGG(Ce)) scintillators was simulated using Geant4．The dependence of angular resolution of such a Compton camera on system geometry, detector energy resolution and Doppler effect as a function of scattering angle was analyzed theoretically．The overall angular resolution is better than 10° when the scattering angle is in the range of 5°~90°．Coincidence events was acquired under 5 different circumstances: ideal case with accurate position and energy detection without considering Doppler effect; only considering system geometry effect; only considering detector energy resolution effect; only considering Doppler effect; taking all the three effects into account．Images are reconstructed using a filtered back projection algorithm running on OpenCL platform when the layer distance of the Compton camera is between 6.0~16.0 cm．Results show that the system geometry effect is the main factor on the angular resolution for the Compton camera simulated．