Abstract: The chirp parameter of modulators is an important index of performance evaluation in fiber-optic communicalion system. We deduced the expression of the chirp parameter of LiNbO_3 external modulator with Mach-Zehnder structure. It shows that frequency chirp in this structure modulator is not equal to zero and the effect of bias voltage on the chirp parameter can be eliminated. The effect of DC extinction ratio, applied electrical signal and DC bias voltage was analyzed. It shows that negative chirp can improve the dispersion characteristics of system.