The purposes of this study were to (1) develop a worksheet that enriched with socio-scientific issues, and (2) examine the effectiveness of the developed worksheets for improving high school students' conceptual understanding of rate reaction and thermochemistry. This research implemented the 4D research and development models that consist of four phases: define, design, develop, and disseminate. Research participants consisted of 132 students from a public high school in Ambon city. The instruments were the validation sheet, questionnaire, observation sheet, and the conceptual chemistry test. The research data consisted of qualitative and quantitative data and analysed using statistics and qualitative descriptions. This study found that the developed socio-scientific issues-based worksheets were valid, practical, and effective in improving high school students' conceptual understanding of reaction rate and thermochemistry. It is necessary to encourage chemistry teachers to explore other socio-scientific issues to teach in rate reaction and thermochemistry.