There has been a range of online Test of English for International Communication courses in Thailand. However, the courses lack interaction, and learners do not have enough opportunities to contact tutors when they have problems. Therefore, the interactive web-based Test of English for International Communication tutoring course was developed in this study to promote interaction in online learning. This study investigates the effects of the course on students' English grammar proficiency and explores students' opinions on the course. The participants were 40 Thai students enrolling in the course. This research employed the mixed-methods approach. The effects of the instruction on grammar proficiency were assessed by pre-and posttests. Students' opinions were explored through reflective journals, questionnaires, and semistructured interviews. The results show that students' grammar proficiency significantly improved after taking the course. All students had a positive attitude towards the course. They perceived a high level of instructor-student interaction, leading to an increase in learning motivation and eagerness to complete the course. Although student-student interaction was in a slightly lower degree, the students had a positive impact on one another. Besides, considerable course flexibility facilitated active learning and increased students' satisfaction. This course also encouraged autonomous learning as the students perceived a sense of responsibility for their study. Extrinsic incentives effectively motivated students to become more active and participate in activities. This study provides a guideline of the teaching approaches to incorporate the concept of interaction into web-based instruction to improve students' grammar proficiency.