Metadiscoursal features are important in teaching and assessing argumentative writing. However, when analyzing some samples of students' writing at tertiary level, it is observed that there is a lack of many metadiscoursal features in students' argumentative writing. For this reason, the current study investigated the impact of the professional development training on teaching and assessing metadiscoursal features in argumentative writing. Using multiple rigorous instruments, the researchers tested the efficacy of the professional development training in teaching and assessing metadiscoursal features. The results showed that the professional development training developed teachers' knowledge, teaching performance, assessment of metadiscoursal features in argumentative writing, and improved students' argumentative writing. Findings highlight the metadiscoursal features relevant for designing assessment instruments for assessing voice, cohesion, and organization in argumentative writing. Furthermore, the present study can serve to provide some new insights about metadiscourse as a target area for teaching and assessment of writing.