I will discuss the NJL model at finite chemical potential in a constant magnetic fields.By using the Schwinger proper time method we calculatetwo-point functions for a scalar and a fermion. In the leading order of $1/N$-expansion the effective potensial of the NJL model is obtained by integrating the two-point functions. I will show the explicit expression of the effectivepotential. Evaluating the effective potential numerically,I will find the phase structure of NJL model at finite chemical potentialin a constant magnetic fields.