Purpose:Although several magnetic resonance imaging (MRI) studies have been conducted in pediatric obsessive-compulsive disorder (OCD), the brain microstructure abnormalities in OCD, especially in children, are not yet well characterized. We aimed to identify microstructural gray matter (GM) abnormalities in the early stage of pediatric OCD and examine the relationship between these structural abnormalities with clinical characteristics. Methods and Materials:Examinations of 40 first-episode, treatment-naive pediatric OCD patients without any comorbidities and 40 matched healthy controls (HCs) were performed with 3.0 T MRI. Voxel-based morphometry (VBM) following Diffeomorphic Anatomical Registration using Exponentiated Lie algebra (DARTEL) was used to conduct voxel-wise tests for group differences in regional gray matter volume (GMV). Results:Compared to HCs, the patient group exhibited more GMV in the bilateral putamen and left orbitofrontal cortex (OFC) and less GMV in the left inferior parietal cortex (IPL). The GMV alternation in the right putamen of OCD patients was positively correlated with HAM-A scores, while the GMV alternation in the left IPL exhibited a trend to negatively correlate with HAM-A scores. Conclusion:Our current results suggest that the gray matter abnormalities were defined in the early stage of pediatric OCD. Moreover, these findings provided further evidence of brain GM abnormalities that are not only present in the classical fronto– striatal–thalamic circuit but also in the default mode network, which may represent the interaction of abnormal functional organization of both network in pediatric OCD.