UNLABELLED: Children with autism spectrum disorder (ASD) exhibit diverse language abilities, yet magnetic resonance imaging (MRI) studies have not systematically examined white matter microstructural differences based on language difficulties. This study aimed to use diffusional kurtosis imaging (DKI) and tract-based spatial statistics (TBSS) to investigate white matter variations in boys with ASD and explore their relationship with language abilities. The study included 61 boys with ASD and 30 typically developing (TD) peers. The ASD group was divided into mild (n = 28) and severe (n = 33) language difficulties subgroups based on the Griffiths Development Scales-Chinese Edition (GDS-C) scores. DKI data were collected for all participants, while clinical symptoms were assessed in the ASD group using the GDS-C and Autism Behavior Checklist (ABC) scales. Correlation analyses examined the relationship between diffusion metrics and clinical scale scores in ASD. Compared to TD peers, children with ASD exhibited significantly reduced white matter microstructural integrity in the corpus callosum (CC), fornix, bilateral corona radiata (CR), inferior longitudinal fasciculus (ILF), inferior fronto-occipital fasciculus (IFOF), and superior longitudinal fasciculus (SLF), with more pronounced reductions in the severe language difficulties subgroup. Additionally, in the mild language difficulties subgroup, mean kurtosis (MK) in the left CR was positively associated with GDS-C language subscale scores. In the severe language difficulties subgroup, MK in the right IFOF showed a positive association with GDS-C language subscale scores, while kurtosis fractional anisotropy (KFA) in the CC was negatively associated with ABC language subscale scores. CONCLUSION: These findings suggest that reduced white matter microstructural integrity in these tracts may be an important neurobiological factor associated with language difficulties in boys with ASD. Additionally, boys with severe language difficulties may exhibit more distinct and widespread white matter differences compared to those with mild difficulties. WHAT IS KNOWN: • Children with ASD often present with varying degrees of language difficulties; language delay is an early and sensitive indicator of ASD. • Previous DTI studies have identified white matter abnormalities in ASD, particularly in tracts related to language processing. WHAT IS NEW: • DKI analyses revealed graded white matter alterations in ASD children based on the severity of language difficulties. • Microstructural changes in specific language-related white matter tracts were significantly associated with language abilities.