Abstract: In order to study the character of free energy density F of some material with an asymmetry of bias exchange of spontaneous magnetization, Landau theory of phase transition is adopted to study the asymmetry. Symmetric free energy density F in one-dimension is expanded into the even power series of spontaneous magnetisation intensity M( T). As for asymmetric F, only M³(T) term is put into consideration, thus the asymmetric M-T and ( F-_g0 )-M curves with the character of second-order phase transition are obtained near the transition temperature T_c between the ferromagnetic and paramagnetic states. The qualitative relation of expansion coefficient of the power series of F is: g₂ = γ ( T- T_c ), in which g₃ decreases and g₄ increases in nonlinearity with the decrease of temperature.