The purpose of this study was to use a retrospective nonlinear distortion correction technique and evaluate the changes in DTI metrics in areas of interest in and around GBM tumors. A total of 24 histologically confirmed GBM patients with pre-operative 20-direction DTI scans were examined. Variability in apparent diffusion coefficient (ADC) and fractional anisotropy (FA) in normal tissue before and after distortion correction were examined. Changes in mean, median and variance of ADC and FA in contrast enhancing and T2/FLAIR ROIs were also examined with and without distortion correction. Results suggest the intra-subject SDs of ADC and FA decreased in normal tissue after the application of distortion correction (P < 0.0001). FA mean and median values decreased after distortion correction in both T1+C and T2 ROIs (P < 0.017), while ADC mean and median values did not significantly change except for the median ADC in T1+C ROIs (P = 0.0054). The intra-subject SD of ADC and FA values in tumor ROIs changed significantly with distortion correction, and Bland-Altman analysis indicated that the bias and the SD of the bias of these intra-subject SDs were larger than those of the mean and median terms. Additionally, the means of the two curves of a double Gaussian fit to the histogram of ADC values from T1+C ROIs, ADCL (mean of lower Gaussian) as well as ADCH (mean of the higher Gaussian) were found to change significantly with distortion correction (P = 0.0045 for ADCL and P = 0.0370 for ADCH). Nonlinear distortion correction better aligns neuro-anatomical structures between DTI and anatomical scans, and significantly alters the measurement of values within tumor ROIs for GBM patients.