The aim of this study was to determine the effectiveness of the 6+1 trait writing model on ESP university students' critical thinking and writing achievement. It was assumed that students who receive training using the 6+1 trait writing model would reveal greater gains in critical thinking and writing achievement. Six instruments -designed by the researchers- included: (critical thinking skills checklist, writing skills checklist, critical thinking skills test, writing skills test and scoring rubric for critical thinking and analytic scoring rubric for writing) were used for data collection. Results revealed that the traditional method used to teach writing is not as effective as the 6+1 trait writing model that developed critical thinking and writing achievement. The experimental group outdid their counterparts in the control group in critical thinking and writing performance test scores.