Purpose: Sentence diversity is a measure of early language development that has yet to be applied to individuals with autism spectrum disorder (ASD). The primary aim of this study was to identify whether children with ASD show change in sentence diversity over 6 months of treatment with Naturalistic Developmental Behavioral Intervention (NDBI). The secondary aim was to examine possible predictors of changes in children's sentence diversity, including caregiver use of NDBI strategies, naturally occurring instances of caregiver "Toy Talk," and child characteristics. Method: Fifty children with ASD (ages 2-4 years) and their caregivers, who were receiving NDBI, engaged in two 10-min video-recorded play interactions, 6 months apart. Child speech was transcribed and coded for sentence diversity. Caregiver input was transcribed and coded for naturally occurring "Toy Talk." Zero-inflated negative binomial mixed models were used to explore predictors of change in child sentence diversity. Results: Children's sentence diversity improved over time. Changes in caregiver NDBI strategy use and caregiver baseline "Toy Talk" were significant predictors of changes in sentence diversity, as were baseline age, nonverbal ratio IQ, and child sex. Additionally, a significant interaction of caregiver baseline "Toy Talk" and change in caregiver NDBI strategies emerged; the effect of caregiver baseline "Toy Talk" on children's sentence diversity change was stronger when NDBI strategy use improved. Conclusions: Sentence diversity is a developmentally sensitive measure of language development in ASD. NDBI strategies that facilitate reciprocal social communication, combined with input composed of declarative sentences with noun or third-person pronoun subjects, may provide optimal support for children's sentence development.