The present study explores the factors that shape learner engagement in writing tasks and the role that output-based instructions could perform in elevating the level of engagement. In so doing, to develop a measure for evaluating learner engagement in writing tasks, a pool of eight university teachers were interviewed and five university students were participated in think-aloud protocol as well as a total of 139 English-major university students were asked to complete the newly-developed inventory. The result of inter-coder reliability was acceptable and Structural Equation Modeling (SEM) provided support for the factor structure of the measures. The final validated inventory comprised four factors and 23 items. Following this, the findings obtained from an experiment on 31 English-major students revealed that both types of task-based collaborative instructions including debating and dictogloss could elevate the level of students’ engagement in writing tasks. More specifically, the statistical analyses indicated that the debate-based instruction could increase the students’ engagement in writing tasks more than the dictogloss instruction. In the end, the linkage between task-based collaborative output activities, engagement in writing tasks and engagement components were discussed, and the pedagogical implications were offered based on the results of the study.