DFT-B3LYP and CASSCF calculations have been performed using a 6-31G* basis set to study photocycloaddition reactions of biradicals, generated by irradiation of 6-amino-2-(3-thienoyl)-1,4-benzoquinone, with ethylene. The calculated parameters of biradicals and transition states have also been compared with ground state parameters to completely elucidate the reaction mechanism of [2 + 2] and [3 + 2] photocycloaddition reactions. Preference for a particular cycloaddition pathway is ascertained by the relative stability between initially formed triplet biradical and another triplet biradical formed by hydrogen shift.