Using exclusively H₂O₂ as an additive, facile morphology control over CaCO₃ crystal aggregation was realized readily by changing the concentrations of H₂O₂ and the pH values of the crystallization system. The ionization of H₂O₂ to [H–O–O]⁻ and [O–O]²⁻ and the generated gas molecules played significant roles in modifying the crystal morphologies. Based on the CaCO₃ particles morphology through different aging times, a mechanism about the formation of CaCO₃ crystals in the presence of H₂O₂ was proposed.