The aim of this study was to investigate the effect of developed collagen (Co) hydrogel (CH), powder-mixed hydroxyapatite/collagen (HA/Co) hydrogel and in situ synthesized HA/Co (In/HA/Co) hydrogel with or without mesenchymal stem cell (MSC) and platelet-rich plasma (PRP) on the regeneration of fullthickness critical size bone defect in the rabbit animal model. In the first step of this study, the scaffolds were synthesized and characterized using FTIR spectroscopy, X-ray diffraction, and scanning electron microcopy. In the second step or animal study, the radial bone defects were filled with the synthesized scaffolds with and without MSC and PRP. One hundred sixty one year-old New Zealand white male rabbits were randomly divided in 16 groups of 10 rabbits including control