A highly ordered, uniformly aligned nanostructure with good crystallinity was first achieved on a P3HT block copolymer possessing a low weight fraction (19.4 wt%) of a flexible polyisoprene segment via a mechanical rubbing process without the use of solution based fabrication and thermal annealing. The attachment of the short polyisoprene segment to P3HT would significantly promote the main chain mobility to allow the orientation control of P3HT and of the self-assembled nanostructure by rubbing.