A statistical mechanical perturbation theory has been used to describe discotic nematic-isotropic phase transition for a discotic liquid crystal, hexa(p-octyl phenyl ethynyl)benzene. We have employed the decoupling approximation, which introduces anisotropy in the pair correlation function, to investigate the discotic nematic-isotropic transition properties. The variation of second rank orientational order parameter with temperature has been computed and it shows qualitatively similar behavior to that of experimental studies of Aver'yanov [1]. However, the numerical value of order parameter at the transition found in the present work is larger than that of Aver'yanov's. The various thermodynamic properties have also been calculated at the transition.