Objective: The aim of this study was to evaluate histologically the effect of the Biphasic Calcium Phosphate and HyaluronicAcid on bone regeneration in critical sized defects in tibia of rats. Materials and methods:  The experimental part was carried out on 16 adult male albino rats and their weight almost 250 gm., the animals were grouped into 2 equal groups as the following; group I (control) which had not receive any thing , group II (experimental) which had received Biphasic calcium phosphate plus Hyaluronic acid (BCP+HyA). Specimens were harvested on days 7 and 28 after surgical procedures, prepared and examined histologically by H&E stain, there were wide histological differences between the groups of this study along the different intervals of the study.  Results: The histological results demonstrated that there was obvious retardation in granulation tissue formation, organization and bone formation in the control group I than the other group along the different intervals of this study. Moreover, there was great acceleration in granulation tissue formation, organization and bone formation in experimental group II which received the (BCP+HyA). this enhancement in bone healing process was due to the effect of HyA as it accelerates the bone regeneration by means of cell adhesion, chemotaxis, proliferation and successive differentiation of mesenchymal cells.