Nanocomposites composed of palladium (Pd) nanoparticles, (Pd)(n), and a symmetric diblock copolymer (dibcp) poly(styrene)-block-poly(isoprene) (PS-b-PI) have been prepared via reducing palladium acetylacetonate, Pd(acac)(2), incorporated in the solution cast PS-b-PI films by a thermal treatment. In our earlier work, we found (Pd)(n) are incorporated into both PS and PI lamellae after the reduction. The reduction mechanism of Pd(acac)(2) and the growth mechanism of (Pd)(n) were found to be independent of the reduction temperature, T-r, regardless of T-r being below or above the order-disorder transition (ODT) temperature (T-ODT). However, the reduction and growth rate as well as the concentration of (Pd)(n) in the final stage depend on T-r: The higher T-r is, the larger are the rate and the concentration of (Pd)(n). In this work, the ODT behavior of thus formed nanocomposites after the completion of the reduction at T-r > T-ODT or T-r < T-ODT was studied by small-angle X-ray scattering (SAXS). The SAXS profiles observed during the cooling and heating processes revealed a thermoreversible change of the structures of the nanocomposite without any changes in the size and size distribution of (Pd)(n). The introduction of a very small amount (similar to 1 wt %) of (Pd)(n) efficiently lowers the T-ODT by 3-13 degrees C, broadens the ODT temperature window, and decreases the spacing of the lamellae, D. These changes are attributed to the less stable ordered phase brought by the incorporation of (Pd)(n) compared with the ordered phase of the neat dibcp because the incorporation causes (i) the large interface to volume ratio for the ordered microdomains and (ii) the large distortion of conformations of the dibcp chains in the ordered microdornains. In addition, we found that the larger the amount of (Pd)(n) created in the dibcp matrix is, the lower is T-ODT.