The packing structures and packing effects on excitation energies of thiophene, terthiophene, sexithiophene, and duodecithiophene are studied by employing the molecular dynamic (MD) simulations and the TDDFT/ 6-311++G** calculations. It has been demonstrated by MD simulations that (1) when going from short to long chain systems, the packing structure prefers to be in the π-stacked form and (2) the effective conjugation lengths, ECLs, of terthiophene, sexithiophene, and duodecithiophene chains are 2, 3, and 4, respectively. Accordingly, the comprehensive effects of the interchain distance, R, and ECL on excitation energy can be estimated by a simple formula. The calculated excitation energies of the packing oligothiophenes are in agreement with the experimental data with the average deviation of 0.12 eV.