This study advances the strand of research on motivational instructional strategies, the process-genre writing instructional approach, as well as student engagement and writing achievements. To this end, motivational writing instructional strategies were integrated into the process-genre argumentative writing instructional approach and the impacts on students' writing engagement and achievements were assessed at a university in China. Quantitative data were gathered from 142 undergraduate students employing a pretest-posttest control group design. Additionally, self-report data regarding students' writing engagement and perceptions were collected from experimental groups to examine the contribution of students' perceptions of the motivational process-genre writing instructional approach to their engagement. Moreover, focus-group data were gathered from four highly-engaged and high-achieving students to identify the factors facilitating their engagement. The results showed that the experimental groups wrote significantly better argumentative essays than did the control group, and regression analyses indicated that motivational instructional strategies contributed to students' engagement significantly. Further, the thematic analysis of focus group data suggested that the three major facilitators of students’ engagement were collaborative motivational writing learning strategies, competence-boosting experiences, and active participation in creating learning opportunities. These findings show the paramount significance of incorporating the motivational strategies into the process-genre writing instructional approach. Other pedagogical implications and recommendations for further research are also discussed.