The present study examined the effectiveness of a cooperative skill intervention program using theme-based joint block play for young children. Thirty-two 4--5 years old preschoolers from Shanghai, China, were assigned to the experimental group while thirty-two children comprised the comparison group. The experimental group was engaged in an 8-week theme-based joint block play program, whereas the comparison group received no intervention but engaged in free block play during school time. Baseline, post-intervention and 2-month follow-up measures included the Deutsch & Krauss Trucking game and observations of children's cooperative skills in pretend activities that occurred in the preschool classroom. Results indicated that the experimental group achieved significantly higher scores in the trucking game, and was observed to endorse significantly less mandatory strategy and more prosocial strategy in pretend play activities than the comparison group. The findings suggest that theme-based block play activities can enhance children's cooperative awareness and skills effectively.