Generally, the various types of intensity distribution found in the vibronic transitions of diatomic molecules are explained using Franck-Condon principle and Condon approximation. Based on the Condon approximation, the electronic transition dipole moment is independent of inter-nuclear distance. In general, this approximation is relatively good enough (not always) for analyzing the electronic spectra of diatomic molecules. For more accurately study of the intensity distribution of vibronic transitions, the variation of the electronic transition dipole moment with inter-nuclear distance should be considered. In this work, a computer program has been written in Maple 9.5 software which is able to analyze the electronic spectra of diatomic molecules with vibrational structure by considering the variation of the electronic transition dipole moment with inter-nuclear distance. This program is able to calculate the spectroscopic parameters (vibrational frequency, unharmonicity and equilibrium distance) of the electronic excited state form the experimental electronic spectrum. The electronic transition dipole moment is considered as a polynomial of inter-nuclear distance which its parameters are adjustable in the program. Using this program the accuracy of Condon approximation for the O 2 ( 3 ? - g ? 1 ? u ) and CO ( 1 ? + ? 1 ?) electronic transitions was confirmed. In another part of this thesis, the 1s photoelectron spectrum of O 2 molecule was analyzed using the program. In this regard, the equilibrium inter-nuclear distance of the 4 ? - and 2 ? - of O 2 + was obtained for the first time. In addition, it was determined that the electronic transition dipole moment varies linearly with inter-nuclear distance (r-centroid approximation) for 4 ? - ? 3 ? g - transition and Condon approximation is acceptable for . 2 ? - ? 3 ? g transition