It is an interesting phenomenon for natural organisms to have control over the shape and size of inorganic nanocrystals and to arrange them into polycrystalline structures. This phenomenon has been driving many attempts to mimic the biomineralization process for synthesizing novel materials. In the present work, novel polycrystalline spindle-like calcite crystals consisting of nanocrystals have been synthesized via a phase transformation process under a Langmuir monolayer of bovine serum albumin (BSA) at room temperature. The polycrystals are composed of hundreds of well assembled calcite nanoneedles consisting of orderly oriented and aggregated nanocrystals transformed from amorphous phase. The evidence for the phase transformation process has been observed in detail by structural characterization of the products at different growth stages. It has been found that the products are evolved from amorphous particles to spindle-like polycrystalline particles. The mineralization process and the interaction between the inorganic and bioorganic components are discussed in relation to relevant protein-mediated nucleation models of biomineralization. Hopefully, the present research is to help understanding the protein-directed formation process of complex structures as well as biomineralization mechanism. (C) 2011 Elsevier B.V. All rights reserved.