BACKGROUND AND PURPOSE To evaluate the time periods of Wallerian degeneration (WD) in which the diffusion parameters of ipsilateral corticalspinal tract (CST) can be used to predict the motor function outcome after brain infarction. METHODS This retrospective study classified 48 diffusion tensor imaging patients with WD along CST into four groups based on the following time points after stroke onset, Group 1: within the first 2 weeks; Group 2: from 3 to 4 weeks; Group 3: from 5 to 14 weeks; Group 4: after 14 weeks. The apparent diffusion coefficient (ADC), fractional anisotropy (FA), and their ratios (=ipsilateral diffusion value/contralateral value) of cerebral peduncle were evaluated. The correlation between imaging parameters in each group and the motor function scores appraised at 8 months after stroke onset were assessed. RESULTS There was no evident correlation of FA ratio (rFA) in Group 1 with motor function score (P= .05). The rFA and FA correlated with motor function score in other groups (P < .001 in each group). The ADC ratio and ipsilateral ADC value only showed significant correlation of with motor function score in Group 4 (P= .016 and .029, respectively). CONCLUSIONS The rFA and ipsilateral FA values after 2 weeks of stroke onset correlate with the motor function outcome.