A roll angle measurement system using a transverse Zeeman laser is presented in this paper. It has a large measurement range and its sensitivity is nonlinearly enhanced. The transverse Zeeman laser provides a plane polarized light, which is changed to be slightly ellipse polarized by a quarter wave plate to enhance the sensitivity nonlinearly. Then, the beam passes a analyzer and the measurement signal is detected by a photo-diode. By measuring the phase difference between the beat signal and the reference signal, the value of the rolla angle of the analyzer is got. By employing the phase detector that has the resolution of 0.01°, a sensitivity of 0.6″ in roll angle of the measurement system can be reached.