Introduction: A decrease in pH of the oral cavity will cause demineralization which may lead to loss of minerals from hydroxyapatite in teeth. The mineral loss can be replenished by applicating a remineralization agent such as cocoa paste. Cocoa beans (Theobroma cacao L.) contain theobromine that allows calcium and phosphorus to bind together to form crystals of hydroxyapatite. However, the application duration of theobromine as remineralization agent is not clearly established. The purpose of this study is to determine the effect of application duration of cocoa paste on surface hardness of tooth enamel. Methods: Enamel samples from premolar teeth (n= 32) were demineral-ized and divided into 4 groups. Enamel hardness was measured by Vickers Hardness Tester. Remineralization was performed using 5% cocoa paste in various duration application: group A (5 minutes), B (15 minutes), C (30 minutes), and D (60 minutes). The surface hardness of enamel was again measured following treatment. Results: Application of cocoa paste for 30 min has indicated the highest surface hardness of the enamel while application for 5 min was the lowest level surface hardness of the enamel. The result of one-way ANOVA test showed that there were signif-icant differences of enamel hardness before and after treatment with cocoa paste. Tukey post hoc test also showed that there were significant differences of enamel hardness between group A and other groups, also between group B and C. Conclusion: The present study showed 5% cocoa paste have a significant effect in increasing enamel surface hardness and the optimal application time is 30 minutes.