The induction period, t(ind), of CaCO3 precipitated from an aqueous CaCl2-Na2CO3 solution is investigated both experimentally and theoretically. The t(ind) of primary nucleation is measured by applying a conductivity method and estimated by using the rapid coagulation theory. By using a novel data acquisition system in the experiment, t(ind) can be easily estimated from the desupersaturation curve, which is a plot of solution conductivity against time. The desupersaturation curves obtained at various levels of supersaturation reveal the important features at different desupersaturation stages, including clusters coagulation, formation of critical nuclei, growth of critical nuclei, detection of visible crystals and equilibrium stage. These individual stages are affected greatly by the initial supersaturation of solution. An oscillation in conductivity, which arises from the fluctuation of solution concentration, is observed between t(ind) and t(v), t(v) being the instant that visible crystals are detected. A comparison between theoretical and experimental results, which include the data obtained in this study and those reported in the literature, shows that the present model is satisfactory for a wide range of supersaturation, and unifies the homogeneous and heterogeneous mechanisms of nucleation. The results also imply that the interparticle forces between clusters are important for the formation of critical nuclei in the primary nucleation regime.