Four bis[1]benzothieno[6,7- d:6',7'- d']benzo[1,2- b:4,5- b']dithiophene (BBTBDT) derivatives bearing substituents on the molecular long axis were synthesized, and their transistor performance was evaluated. Among the obtained compounds, OFET devices based on the 2,9-diphenyl-substituted derivative (1d) on a β-PTS-modified Si/SiO2 substrate yielded the best morphological and crystalline structures, resulting in the highest hole mobility, as high as 0.16 cm2 V-1 s-1, and a low threshold voltage of -8 V. In the solid state, 1d formed a highly ordered and crystalline edge-on structure, which facilitated effective carrier transport. The detailed structure-property relationships were also disclosed by GIWAXS analysis, atomic force microscopy measurements, and theoretical calculations.