Time-resolved small-angle X-ray scattering (SAXS) measurements were performed to study in-situ formation of palladium (Pd) nanoparticles in block copolymer microdomains. The nanoparticles were reduced in a reaction medium containing a fixed composition of polyisoprene-block-poly(2-vinylpyridine) (PI-b-P2VP), palladium acetylacetonate, Pd(acac)2, and benzyl alcohol as a solvent and a reducing agent. The polymer concentration is high enough so that it forms swollen ordered lamellae before reduction. The reduction was induced by a rapid temperature jump, and the polymer concentration was kept constant in a sealed sample cell. It was clearly elucidated that the formation of Pd nanoparticles occurred in the templates of the swollen lamellar microdomains. By fitting the theoretical scattered intensity for spherical particles to the observed SAXS profiles, we analyzed time evolution of mean radius, standard deviation of radius, and total volume of Pd nanoparticles in concentrated PI-b-P2VP solutions after T-jump. These results and the information obtained in our previous communication indicate that Pd(II) ions before reduction distribute equally well in both the PI and P2VP lamellae; after T-jump Pd nanoparticles form and grow only in the P2VP lamellae after an induction period, and the growth stops when the size reaches a steady value; the final size and total volume of Pd nanoparticles created depend on the reduction temperature. The formation of I'd nanoparticles in a P2VP homopolymer solution was also investigated by the same method as that employed for the block copolymer solution in order to compare the two results with and without the template and hence to explore roles of the template on the nanoparticle formation.