The population analysis along the intrinsic reaction coordinate (IRC) has been performed, taking 1,3-dipolar cycloaddition as an example. By calculating the derivatives of the Mulliken atomic charge and the Giambiagi bond order with respect to nuclear motions analytically, a decomposition of the derivatives into two components, the density derivative term and the overlap derivative term, has been demonstrated. The density derivative term represents the effect of redistribution of electrons due to the nuclear motion. By using this method, the electronic character of asynchronous bond formation in 1,3-dipolar addition between fluminic acid and acetylene has been revealed clearly.