Introduction: Every material must have resistance when applied pressure. One of the mechanical properties is sur- face hardness, which is a very important mechanical property because it affects the duration of use of a material. The use of a temporary crown is needed to protect the teeth after preparation before insertion of a permanent crown. The material often used to make temporary crowns is self-cured acrylic resins. However, self-cured acrylic resins have the disadvantage of being monomer residues and suboptimal hardness in thin areas. Hydroxyapatite produced by the precipitation method is one of the materials with good biocompatibility and has been tested as a reinforcing material. High hydroxyapatite content, economic value and processing which is not yet maximal in skipjack fish bones are a concern that needs to be developed. The purpose of this study was to determine the effect of addition of hydroxy- apatite of skipjack fish bones on the surface hardness of self-cured acrylic resin. Methods: This type of research is an experimental laboratory with Post-test-only control group design involving 2 groups, namely the experimental group with the addition of hydroxyapatite concentrations of 2% and 5% and the control group. The sample used was a 9x8x2 mm self-cured acrylic resin plate with a total of 27 samples with 9 samples for each group. Samples were tested with a microhardness vickers tester. Results: the treated group showed greater surface hardness than the con- trol group. the greater the concentration of hydroxyapatite used in acrylic self-cured resin powder produces a greater surface hardness value. Conclusion: hydroxyapatite of skipjack fish bone (Katsuwonus pelamis) has succeeded in increasing the surface hardness of self-cured acrylic resins in temporary crowns.