Research on mathematics teacher knowledge has gained momentum recently; however, research on teacher planning is scarce, especially using lesson plans as the primary data source. This study examines how teachers' lesson plans change as they participate in a professional development, supporting them in implementing the Vietnamese reformed Curriculum 2018. Using a design-based research methodology, we developed a research-informed framework that assesses teachers' lesson plans for developing students' statistical literacy as emphasized in the mathematics Curriculum. Informed by research literature and data collected from teachers, the framework including seven criteria was shared and used by 61 teachers to reflect on the quality of their lessons. The analysis of 38 lesson plans shows all criteria have improved from pre- to post-lesson plans. The teachers included more explicit and comprehensive learning intentions in their post-lessons. Tasks in the post-lesson plans afforded more opportunities for students to develop statistical literacy, helped them engage in the statistical investigation using real data, and included multiple representations. Finally, the post-lesson plans tended to follow the constructivist teaching principles. Implications on ways to develop teachers' planning competence and suggestions for future studies are included.