This study aims to find whether the adapted approaches of writing instructions and alternative assessment could improve the students’ academic writing ability. This classroom-based action research was initiated from the observation and reflection of the researchers as English academic writing instructors in tertiary education. It was observed that students had difficulties improving their academic writing ability. The preliminary investigations revealed that the problem was caused by the dependence on a single approach of instruction and summative assessment. Adapting the strengths of different approaches of writing instructions: the product approach, process approach, genre approach and process-genre approach, the intervention was designed. Alternative forms of assessment were integrated in the pedagogy for their potentials in promoting learning. Through the steps of plan, act, observe, and reflect of action research, data on benefits and shortcomings of the intervention were analyzed and used in reshaping the instructions. Results from the statistical analysis of the discrepancy between the pre-test and post-test revealed that the intervention significantly improved the students’ academic writing ability. Besides, the findings from multiple sources also showed that the students developed positive opinion towards this pedagogical approach. Finally, this study offers informed knowledge on factors to consider in using the adapted approaches of writing instructions.