In this paper we have derived, in the usual molecular field approximation, an equation for the thermodynamic potential for determining the critical temperatures of two phase transitions in a mixture of ellipsoidal and spherical molecules. These molecules are dispersed on b.c.c.-lattice and their ratio of concentrations is different form 1:1. We have also obtained the bound lines of the orientational and positional transitions in the temperature-concentration diagram for different values of molecular parameter (of ellipsoidal molecules) and interaction parameters. (author). 15 refs, 2 figs, 1 tab.