Purpose: To test whether diffusion tensor imaging could evaluate potential motor capability of patients with chronic cerebral infarction. Methods: We used constraint-induced movement therapy (CIMT) as a rehabilitation, which reveals potential motor capability. We also investigated the relationship between the outcome of CIMT and the ratio between fractional anisotropy values (rFA) in affected and unaffected sites of the corticospinal tract before CIMT. Imaging was performed in cerebral infarction patients (n = 14) and the rFA of the posterior limb of internal capsule (PLIC) was measured before CIMT. Patients were evaluated before and after CIMT using the Fugl-Meyer (F-M) assessment, Wolf Motor Function Test, Action Research Arm Test, and Motor Activity Log and association between PLIC- rFA and these scores was determined. Results: All patients showed an improvement in mobility following the 10-day CIMT session. Strong positive correlation was found only between F-M after CIMT and PLIC-rFA (r = 0.8098, p = 0.0004). A strong linear relationship was observed after CIMT. Conclusion: These data support PLIC-rFA as a new marker of the CIMT-induced improvement in motor function (F-M).