A variety of synthetic materials are currently in use as bone substitutes, among them a new calcium phosphate-based multichannel, cylindrical, granular bone substitute that is showing satisfactory biocompatibility and osteoconductivity in clinical applications. These cylindrical granules differ in their mechanical and morphological characteristics such as size, diameter, surface area, pore size, and porosity. The aim of this study is to investigate whether the sizes of these synthetic granules and the resultant inter-granular spaces formed by their filling critical-sized bone defects affect new bone formation characteristics and to determine the best formulations from these individual types by combining the granules in different proportions to optimize the bone tissue regeneration. We evaluated two types of multichanneled cylindrical granules, 1 mm and 3 mm in diameter, combined the granules in two different proportions (wt%), and compared their different mechanical, morphological, and in vitro and in vivo biocompatibility characteristics. We assessed in vitro biocompatibility and cytotoxicity using MC3T3-E1 osteoblast-like cells using MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide) assay and confocal imaging. In vivo investigation in a rabbit model indicated that all four samples formed significantly better bone than the control after four weeks and eight weeks of implantation. Micro-computed tomography analysis showed more bone formation by the 1 mm cylindrical granules with 160 ± 10 μm channeled pore and 50% porosity than the other three samples (p<.05), which we confirmed by histological analysis. [ABSTRACT FROM AUTHOR]