Abstract: The thermal decomposition behaviors of three kinds of derivatives, including 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO), 10-(2, 5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO-HQ), 6H-dibenzc, e1, 2oxaphosphorin, 6’6-(1, 2-phenethyl)bis-, 6’6-dioxide (DiDOPO), were studied. And effects of these DOPO derivatives on the thermal degradation behavior, flame retardancy and mechanical properties of poly(lactic acid) (PLA) were comparatively studied. TG results show that the initial decomposition temperature (T_5%) of DOPO is 154℃, and an increased T_5% of DOPO-HQ of 342℃ is displayed. In addition, T_5% of DiDOPO reaches to as high as 363℃, which is higher than that of DOPO and DOPO-HQ. As for PLA composites, T_5% reveals an increase trend as follow: T_5%(DOPO/PLA, 273.5℃) <T_5%(DOPO-HQ/PLA, 321.5℃) <T_5%(DiDOPO/PLA, 333.8℃). T_5% shows an progressively increase trend with the increased thermal stability of flame retardants. Furthermore, the thermal degradation kinetics results indicate that the thermal degradation activation energy of the composites material is improved. The thermal degradation of DOPO derivatives/PLA composites was investigated by thermogravimetric-infrared spectroscopy analysis (TG-IR) and pyrolysis-gas chromatography/mass spectrometry (PY-GC/MS). It shows that the three flame retardants mainly exert their gas phase flame retardant effect by generating phosphorus oxygen free radicals, and the characteristic groups of DOPO-HQ and DiDOPO may play a role in the condensation phase. All the three systems have attained the V-0 rating (UL-94), what’s more DOPO/PLA dripping is the most serious, on the contrary the dripping of DiDOPO/PLA is suppressed. Furthermore, the mechanical test results indicate that the tensile strength of DOPO/PLA decreases by 83.1% compared with pure PLA, while DOPO-HQ/PLA and DiDOPO/PLA only drops by 14.2% and 15.6%, respectively.