The novel calcium carbonate (CaCO3) morphology, twin-sphere with an equatorial girdle, has been obtained under the control of poly(L-lysine) (PLys) through gas-diffusion method. The effect of the concentration of calcium cation and PLys, the reaction time, and the initial pH value are investigated, and various interesting morphologies, including twinsphere, discus-like, hexagonal plate, and hallow structure are observed by using scanning electronic microscopy. Laser microscopic Raman spectroscopy studies indicated that all these CaCO3 are vaterite. A possible mechanism is suggested to explain the formation of the twin-sphere based morphologies according to the results. It is proven that alkaline polypeptides can control the mineralization of CaCO3 precisely as the reported acidic polypeptides and double hydrophilic block copolymers.