This study examined the effect of the Science-Writing Heuristic (SWH) approach on 5th-6th grade students' critical thinking skills compared to a control group. Students in the SWH group received professional development on the SWH approach, which integrates literacy into science lessons through claim and evidence writing. Both groups took the Cornell Critical Thinking Test before and after the intervention. Results showed greater gains in critical thinking scores for the SWH group compared to the control group in both year 1 and year 2, with moderate to large effect sizes. The study aims to publish results and allow for future statistical analysis of subgroup differences.