  The effect of finite isospin chemical potential to the effective masses of
the mesons at finite temperature is investigated in the framework of the O(4)
linear sigma model with explicit chiral symmetry breaking. We present a
mechanism to include the isospin chemical potential in the model. By using the
Cornwall-Jackiw-Tomboulis method of composite operators, we obtain a set gap
equations for the effective masses of the mesons and get the numerical results
in the Hartree approximation. We find that the introduction of the chemical
potential only affects the mass of the charged pions and sigma, while there is
almost NO effects on the mass of neutral pions.
