〔Abstract〕 Objective To analyze the distribution characteristics of free water ( FW) and FW-corrected fractional anisotropy (FAt) in the white matter of the brain in patients with anti-N-methyl-D-aspartate receptor ( NMDAR) encephalitis and to explore their correlation with cognitive function．Methods A total of 38 patients with anti- NMDAR encephalitis and 30 controls were recruited from three hospitals in Hefei．Diffusion tensor imaging data and neuropsychological assessment results were collected．Tract-Based Spatial Statistics was applied to compare group differences in FW and FAt across the whole brain white matter．Correlation analyses were further performed to ex- amine the relationships between FW/ FAt metrics and cognitive function．Results Compared to the control group， patients with anti-NMDAR encephalitis showed significantly lower scores on the montreal cognitive assessment，im- mediate recall，delayed recall，and the verbal fluency test (all P＜0. 05) ，as well as significantly longer comple- tion times for color naming and word reading tasks in the stroop color word test (all P＜0. 05) ．Diffusion tensor im- aging analysis revealed significantly elevated FW and reduced FAt in widespread white matter regions in the patient group (all P＜0. 000 1) ．Further correlation analysis showed that increased FW was positively associated with the completion time of the color-switching condition in the color digital trail making test (P = 0. 044 ) and with the difference between color-switching and number sequencing conditions ( P = 0. 016 ) ，while negatively correlated with semantic fluency scores (P = 0. 002) ．Additionally，FAt was positively associated with delayed recall perform- ance (P = 0. 012) ，and negatively correlated with the completion times for color naming (P = 0. 018 ) and word reading (P = 0. 046) tasks in the SCWT．Conclusion Patients with anti-NMDAR encephalitis exhibit significantly elevated FW and significantly reduced FAt in white matter tracts，both of which are closely linked to cognitive im- pairment．