The mechanical properties of monofilament of imports and domestic poly-phenylene benzodiazole (PBO)fiber,and aramid fiber were measured by constant velocity tensile tester. The effects of different preparation conditions and test conditions on the tensile properties of organic filaments were investigated. The main factors included loading rate,gauge length,fiber moisture absorption and thermal aging temperature. The results show that the tensile strengths of the four fibers calculated by Weibull statistical method decrease gradually with the gauge length increasing from 5 mm to 60 mm. With the loading rate increasing from 5 mm/min to 200 mm/min,the tensile strengths of the four kinds of fibers show first increase and then decrease trend of changes. With the increase in moisture absorption,the strengths of imported PBO fiber and aramid fiber both decrease gradually. As the accelerated aging temperature increases,the imported PBO fiber and aramid fiber show reduced tensile strength,which decrease by 39.1% and 51.6% respectively after heat treatment at 300℃ for 40 h.