Nearby electron/positron accelerators, mostly Pulsar Wind Nebulae (PWNe), have been pro- posed as potential origins of the positron excess above 10 GeV. The HAWC Observatory reveals two very extended sources spatially coincident with two nearby middle-aged pulsars: Geminga and PSR B0656+14, suggesting ultra-relativistic electrons/positrons accelerated in our backyard. Morphological studies on these two PWNe provide a constraint on the diffusion coefficient at HAWC energies. In this poster, we will present the model development and morphological stud- ies on these PWNe, and the derived diffusion coefficient that best fits the data.