〔Abstract〕 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.