Polarization Dependent Loss (PDL) accumulated by devices such as optical modulators,switches,filters,isolators in fiber-optic communications system remarkably degradated the transmission property of the whole link so that PDL of passive components became an important specification parameter. PDL of M-Z modulator influenced by stress between InP/In_1－X Ga_XA_S1－y P_y-MQW and its substrate and hence the Polarization Mode Dispersion (PMD) represented by Differential Group Delay(DGD) were first experimently observed by Agilent 81910A Photonic All Parameter Analyzer. The outcome shows PDL and DGD of semiconductor Multiple-Quantum-Well(MQW) are consistent. During the fabrication of semiconductor optical devices,the residual stress between InP/In_1－X Ga_XA_S1－y P_y-MQW and its substrate should be lessened as greatly as possible to avoid its impact on high speed optical devices'performance.