This paper reports a study using a quasi-experimental design to examine whether an academic writing course in English can improve graduate students' academic and popular science writing skills. To address this issue, we designed pre- and post-assessment tasks, an intervention assessment task and a scoring rubric. The pre- and post-assessment tasks included writing in contrasting genres, i.e. an academic paper abstract to assess academic writing and a press release to assess popular science writing. Students' pre- and post-assessment tasks were rated using a newly developed rubric addressing English proficiency, and academic and popular science writing skills. The rubric was based on course material, previous research and a pilot study. The study analysed 177 non-native English science and engineering graduate students' writing in a compulsory Academic Writing in English course at the beginning and end of a 14-week semester. Findings indicated significant improvement in academic and popular science writing, as well as improvement in students' English language proficiency.