We present a novel one-step assembly process that encompasses in-situ formation of silver nanoparticles (NP), directed assembly of block copolymers into a perpendicular lamellar morphology, and NP organization into patterned arrays. This process utilizes organometallic precursors that thermally decompose and form spatially organized NP arrays on the free surface of the block copolymer. As NP concentration increases, the conversion of perpendicular to parallel lamellar morphology at the free surface slows down. A cell dynamical system (CDS) simulation captures the dynamics of this reorganization. The final structure is attractive for producing nanodevices because the surface is decorated by NP that are arranged in parallel arrays with tunable NP separation.