Critical thinking becomes an essential competency for people in the new information age. The purpose of this research is to investigate to what extent writing online reflections using weblogs can promote students’ critical thinking. This research is conducted in a secondary school. After completing 3 weblogs, a focus group interview was conducted to collect their feedback on the design and implementation of the research. The results showed that the students liked the way of writing online reflections. They attempted to make the weblogs interesting, convincing and logical. Also, they liked to view others’ reflections and to give comments. This paper presents the formative evaluation processes and results of the research.