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The effects of the structural symmetry on the geometrical and electronic structures,molecular stacking motifs,carrier injection as well as transport properties for a series of Dithienothiophenes[1](α-annulated[2] and β-annulated oligothiophenes[3])were theoretically investigated by quantum chemical methods.The better structural symmetry facilitates electron delocalization and extends the effective conjugation length of the system,increases the HOMO energy and decreases the LUMO energy,and thus improves the ability of hole and electron injection.More important,it decreases the reorganization energy and promotes the molecular π-stacking.Small reorganization energies and large electronic couplings give rise to their excellent transport properties,which makes the highly symmetrical molecules become a class of promising candidates for excellent organic field-effect transistor(OFET)materials.