A novel 3D diffusion mapping method using RF phase-modulated gradient echo imaging was developed by encoding diffusion weighting into signal phase. We developed closed form signal equations based on configuration theory. A four-pass acquisition and lookup table-based reconstruction is performed to estimate diffusion and T2 simultaneously. Simulation results revealed excellent agreement of signal magnitude and phase between Bloch equation and the developed equations. Phantom and in vivo studies demonstrated the feasibility of the proposed approach. These results demonstrate that the proposed method can enable qualitative and quantitative diffusion imaging with smaller gradient amplitude compared to the conventional diffusion-weighted imaging.