A systematic study was made on the optical absorption spectra of permethyl oligosilanes, CH3[CH3-Si-CH3]NCH3, as a function of their backbone chain length from N = 2 to 16 by using first principles [density functional with local density approximation (LDA)] electronic structure calculations. The general features of the calculated spectra agree with those of experimental measurements. Analysis of the nature of optical transition revealed the existence of two qualitative crossovers at chain lengths of N = 2 and around N = 10 and 12. The first one is associated with the molecular orbitals giving rise to the first ultraviolet absorption peak, while the second one is associated with the crossover from "molecular" to "polymeric" as the oligomers become larger. The broadening of the first absorption peaks is estimated by the vibration modes and electronic relaxation energies in the oligomers. Comparison with calculated results by a model Hamiltonian based on the Sandorfy C model is made and the relative merit of semiempirical and LDA calculations is discussed.