Absorption spectra of trans-stilbene and its substituted form: 4-methyl-3'-azastilbene are calculated using the Intermediate Neglect of Differential Overlap/Spectroscopic (INDO/S) quantum-chemical method. Energy level diagrams are drawn and analyzed and the rate constants of different photophysical processes in the both molecules are obtained. The effect of the nitrogen atom on spectral properties of the trans-stilbene molecule is studied.