CaCO3 polymorphs with rhombohedral, needle-like and tree-like morphologies have been synthesized by a convenient hydrothermal reaction with the absence of additives or elaborating pH control. XRD, SEM, FT-IR and TGA-DTA techniques were used to characterize the structural features and thermal behavior of the experimental products. The experimental conditions were explored systematically and it was found that polymorphs with manifold morphologies of CaCO3 could be conveniently synthesized by controlling the concentration of reactants.