We studied how the palladium nanoparticles, (Pd)(n)s, incorporated into poly(styrene)-block-poly(methyl methacylate) (PS-b-PMMA) block copolymer (bcp), alter its order-disorder transition (ODT). The (Pd)(n)s were incorporated into the lamellar template of PS-b-PMMA by the thermal reduction of Pd(acac)(2), dissolved uniformly in the bcp before the reduction, at 230 degrees C where both the neat PS-b-PMMA and the PS-b-PMMA incorporated with (Pd)(n)s (nanocomposite) are in the disordered state. The ODT behavior of the neat bcp and the nanocomposites are investigated by small-angle X-ray scattering as a function of temperature. We found that: (i) the average size and size distribution of (Pd)(n)s did not change at all during the heating and cooling processes across the ODT; (ii) the incorporation of a very small amount (1 wt %) of (Pd)(n)s significantly increased the ODT temperature (T-ODT) by similar to 8 degrees C and the lamellar spacing D by similar to 1%. These trends were found to be completely opposite to those previously reported on the (Pd)(n)s incorporated in the lamellar template of poly(styrene)-block-polyisoprene (PS-b-PI), in which (Pd)(n)s were neutral to both PS and PI and acted as quench disorders (Zhao, Y.; et al. Macromolecules 2009, 42, 5272-5277). These intriguing effects observed for PS-b-PMMA are attributed to effects of (Pd)(n)s enhancing thermal stability of the ordered lamellae, hence raising the thermal-fluctuation-induced first-order phase transition temperature, T-ODT. These effects of (Pd)(n)s are anticipated to be mediated by relatively stronger attractive interactions of (Pd)(n)s with PS block chains than with PMMA block chains, which increases the net effective segregation power between PS and PMMA block chains and hence stabilizes the ordered lamellae. The selective attractions were experimentally confirmed by (1) a selectively larger incorporation of (Pd)(n)s in PS lamellae (similar to 70%) than in PMMA lamellae (similar to 30%) and by (2) the selective incorporation of (Pd)(n)s in the lamellae being conserved during the cooling and heating cycles of the nanocomposite across the T-ODT as evidenced by the thermo-reversible change in the scattering profiles. The attractive interactions account for the conservation of the average size and size distribution also, i.e., no coarsening of (Pd)(n)s, with the thermal treatments.