The crystallization of calcium carbonate (CaCO3) was investigated using a mineralization system composed of a chitosan membrane and regenerated silk fibroin (RSF). Such a system may resemble the mineralization in molluscs, where chitosan is a derivative of chitin and RSF an analogue of nacreous protein. It was found that the vaterite disks generally formed on the chitosan membrane while the aragonite disks also appeared with changes of pH value or temperature of the solution. The crystallization of CaCO3 in the vicinity of the chitosan mem-brane was much more affected by the environment of crystallization, compared to that in bulk solution. Detailed observation from high-resolution scanning. and transmission electron Microscopy (HRSEM and TEM) showed that these disks consisted of nanoparticles about 20 nm in size, thus suggesting that the accumulation of hybrid CaCO3/RSF nanoparticles induced the formation of crystalline disks on the chitosan membrane.