The goal of this study was to evaluate the function of fibers that connect the hand area of the primary motor and sensory cortices. A 63-year-old male with a metastatic brain tumor located in the left primary motor area underwent awake surgery. During removal of the tumor, the posterior end of the primary motor cortex, including the fibers between the motor and sensory cortex, was resected, as confirmed by 3D reconstruction of fMRI and diffusion tensor imaging (DTI). Preoperative fMRI with right hand clenching revealed activation in the area around the tumor in the primary motor area. Postoperative fMRI with right hand clenching showed posterior shift of the activation to the sensory area. In contrast, 3D reconstruction of fMRI and DTI with left hand clenching showed activation of cortex corresponding to the motor and sensory hand area. Postoperative neurological examination revealed no change in right hand strength, but the patient complained of decrease right hand grasp stability when he concentrated on using his left hand. These findings suggest that the fibers connecting the primary hand motor and sensory areas play a role in the hand grasp stability.