3,7-Didocecy1-2,6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b']dithiophene (1) and its 4,8-didoclecyl isomer 2 were prepared as the representative soluble X- and cross-shaped pi-conjugated oligomer systems to provide insight into the effect of the substitution pattern of the alkyl side chain on electronic properties. The absorption and emission spectra as well as CV data showed the relatively longer effective conjugation of cross-shaped 2. The intrinsic charge-carrier mobilities were then estimated by flash-photolysis time-resolved microwave conductivity (FP-TRMC) method and compared with their top contact FET properties. It was found that, although the TRMC method showed the higher mobility of 2 than 1, the FET performance of 1 after appropriate conditioning and thermal annealing was superior to that of 2. The effective conjugation of cross-shaped 2 is well reflected in the intramolecular mobility of positive holes estimated by FP-TRMC, showing striking contrast to the rather higher mobility of X-shaped 1 observed by PET as well as TOF measurements as the long-range translational motion of the carriers. This strongly suggests that the intermolecular packing of these compounds plays a significant role in the range of hole mobility of