The modulation function of proteins was studied during biomineralization in vitro. Morphological development of calcium carbonate precipitation in collagen solutions at different concentrations has been examined using scanning electron microscope. X-ray diffraction is used to determine the polymorph of calcium carbonate crystals. Its data indicate that only calcite crystals are formed. The experimental results also show that the calcite growth is more and more inhibited as collagen is increasing in concentration. A simple crystal model is employed to explain these phenomena. Different crystal planes with different affinities to collagen play a crucial role in changing the morphology of calcite crystals.