To get insight into the roles of the collagenous matrix in the hard tissue formation, we have been investigating how the property of the collagenous matrix affects the deposition and crystal growth of calcium phosphates in an in vitro system under physiological conditions. In this study, it was examined whether or not octacalcium phosphate (OCP) deposits on the type I collagen fibrils with any correlation to the fibril. Ribbon-like OCP crystals (10-30 μm in length) grew on the Ca-side of the collagenous matrix, while small plate-like OCP crystals (<1μm) grew inside the matrix. OCP crystals grew epitaxially on the collagen fibrils. The major part of crystals grew with the c-axis parallel to the collagen fibers. Others grew with the c-axis parallel to the collagen bands or perpendicular to the fibrils. Observation of the early stage of the crystal growth suggested that nucleation and growth direction of the crystals have correlation with the structure of the collagen fibrils. It was concluded that property of the collagenous matrix and the structure of the collagen fibril play some regulatory roles in the oriented deposition and subsequent growth of OCP on the collagen fibrils.