Seeded nucleation of lithinm carbonate in aqueous solution during reactive crystallization was monitored by FBRM (focused beam reflectance measurement) and PVM (particle video microscope). The impacts of operating variables, such as seed size and loading, stirring speed, on induction period and secondary nucleation were investigated and explained by an adsorption model. The results show that seed surface area plays an important role in secondary nucleation, for more surface area has higher adsorption capacity and consumes more supersaturation on seed growth, thus restrains nucleation better. A method through comparison between pure breakage/attrition and nucleation process was put forward to distinguish attrition-induced and surface-induced nucleations quantitatively, which can reveal the contributions of different nucleation mechanisms. The nucleation processes in different conditions were studied, the principles and valuable experimental data were obtained for seeding approach primarily. FBRM and PVM are useful on-line apparatuses to facilitate seed selection and seeding optimization.