Due to the increasingly serious outbreak of the Covid-19 pandemic, many Vietnamese and foreign educational institutions at different levels have been using a variety of online teaching and learning methods to continue their current training programs while still meet the quality requirements. The blended learning model is considered a firm alternative that helps expand and complement traditional teaching in most colleges or universities. This paper aims to explore the impact of structured online writing activities to the expected learning outcomes of English-major students who participated in the blended academic writing course at level B1. Course designs are based on constructivist theory and activity theory with an emphasis on promoting collaborative and problem-based learning, which primarily takes place on Canvas, one of the most learning management systems at present. Key writing activities include different types of interaction between teacher and student, among students, between student and learning content, between student and technology in order to build good paragraph writing skills. Findings of this study can be applied to other educational context to facilitate traditional writing courses or design blended writing courses, enhance composition skills, communication and interaction skills, as a result, learning quality, experience and outcomes are increased effectively.