Introduction: Prevention strategy is important in caries management to reduce the risk of caries in children. Fluoride intake for a long time can cause an increased incidence of fluorosis. Theobromine is an alkaloid compound of the methylxanthine group found naturally in cocoa beans (Theobroma cacao). It can play a role in increasing enamel surface hardness by exchanging minerals on the enamel surface. Methods: This study used 20 specimens of primary teeth that were embedded in epoxy resin. Caries-like lesion was created on each specimen using 37% phosporic acid for 60 second. Specimens were divided into 5 groups of topical application of theobromine gel with the concentra-tion of 1%, 2%, 3%, 4% and 5% for 4 minutes. Then specimens were immersed in artificial saliva in 60 minutes. The hardness of the enamel surface was tested using Vickers Microhardness Tester. A statistical tes was performed using Kruskal-Wallis test and Mann-Whitney test. Results: The result of the analysis showed a significant increase in enam-el microhardness between different concentration of theobromine gel (p<0,005). The higher the theobromine gel concentration, the greater of increases the enamel surface hardness in primary teeth. Theobromine gel group of 5% concentration gave the highest increase in microhardness compared to the other groups. Conclusion: Theobromine gel concentration of 5% is effective in increasing enamel surface hardness. Theobromine can be used as potential material to prevent caries in children.