Introduction: Loss of teeth for a long time without replacement of teeth will cause changes in the arrangement of teeth and can distrub in speech, mastication, and aesthetic functions. One of the ways to handle this case is the use of dentures. Dentures are generally made of heat cured polymethyl-methacrylate (PMAA) acrylic resin which has a disadvantage of low hardness. This can affect the mechanical properties of heat cure acrylic resin, namely hardness. There must be a way to overcome the low hardness of acrylic resin by adding reinforcing agents to its composition. The material used is hydroxyapatite of skipjack tuna bones. This research is aimed to determine the hardness of heat cured acrylic resin when added hydroxyapatite synthesized from skipjack tuna bones. Methods: The research meth- od used posttest only group design, that was conducted using 27 samples of 60x15x2.5 mm3 heat cured acrylic resin plates. The samples were divided into 3 groups, namely groups with the addition of 0%, 2%, and 5% hydroxyapa- tite each of the polymer’s weight.The hardness of all samples was measured using a micro-Vickers hardness tester. Results: The results showed the average hardness of acrylic resin without addition was 19.57 HV, with the addition of 2% was 22.85 HV and the addition of 5% was 27.07 HV. Based on the results of the analysis, the results of the one-way Anova test showed a significance value of p<0.05. Conclusion: There is an increasing of hardness of heat cured acrylic resin by adding hydroxyapatite powder of skipjack tuna.