Purpose. Online teaching was adopted in Hong Kong and many other places worldwide in compliance with the lockdown protocol during the COVID-19 pandemic. This research explores factors affecting the effectiveness of primary students' online learning as measured by students' responses to a questionnaire designed for this study. Method. 1,810 students from primary schools in Hong Kong from 15 out of 18 districts in Hong Kong completed the questionnaire in 2023. A Questionnaire was developed for this study to measure four factors, namely, family support in providing hardware and software to students, students' technology competence in using computers or iPads for learning, students' engagement in online learning, and online learning effectiveness. Confirmatory Factor Analysis showed that the questionnaire was reliable and valid. Structural Equation Modeling was used to test the hypotheses. Findings. The findings showed that family support in providing hardware and software and students' technology competence significantly affect students' engagement during online learning, and students' engagement significantly positively affects online learning effectiveness. Implications for Research and Practice. The study can cast light on the important relationships of family support and students' technology competence on students' learning engagement and online learning effectiveness. Furthermore, online teaching can be effective for primary students if sufficient support can be available. Copyright © 2024 Ani Publishing.