Organic field-effect transistor (OFET) characteristics of phenanthrodithiophene (PDT)-isoindigo (IID) copolymers with different alkyl chains (12OD and BOBO) as an active layer, and their structure property relationships are described. The fabricated 12OD-based OFET device exhibited hole mobility of 0.16 cm(2)V(-1) s(-1). Grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that 12OD formed highly ordered edge-on structure, whereas the BOBO film showed amorphous-like structure with no obvious pi-stacking crystallites. This may be attributed to a large steric hindrance of branched side chains in the BOBO polymer, which suppresses effective pi-pi overlaps between the adjacent polymer backbones.