Conjugated oligomers form a fascinating class of molecular semiconductors, which open the perspective of control of electronic and structural properties through the variation of their chemical structure. For analysing the correlation between charge transport and structural properties, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal α positions (α,ωDH6T) and as pendant groups in β position (β,β′DH6T). Structural characterizations by X-ray diffraction show that vacuum evaporated thin films of 6T and α,ωDH6T consist of layered structure in a monoclinic arrangement, with all-planar Molecules standing on the substrate, and that a much longer range ordering is observed when passing from 6T to α,ωDH6T, evidencing a large increase of molecular organization at the mesoscopic level. Electrical characterizations also indicate a significant enhancement of anisotropy of conductivity, with a ratio of 120 in favor of the conductivity along the stacking axis for α,ωDH6T. The charge carrier mobility, measured on field-effect transistors fabricated from these conjugated oligomers, also shows a large increase by a factor of 25 when passing from 6T to α,ωDH6T, and reaches a value close to 10-1cm2V-1s-1. In contrast, ββ′DH6T presents very low conductivity and mobility. These observations are attributed to the self-assembly properties brought by alkyl groups in α,ω position, and confirm the large potential of molecular engineering of organic semiconductors.