Abstract: Some influence factors in tensile property test of CF yarn (3K, 6K, 12K) were investigated in this paper. The factors included sizing concentration, sizing type, tension, relative humidity while preparing of specimens, and speed of loading in the test of specimens. The results showed that testing conditions during preparation of specimens and test procedure played important roles in the results of tensile property of CF. When sizing concentration was lower than 25%, CF tensile strength was lower than normal value. Comparing two kinds of resins, epoxy 6101 and epoxy 648, the best CF tensile strength was obtained by using epoxy 6101. Adding appropriate tension to the CF yarn during sample soldifing, the higher CF tensile strength can be obtained .In addition, CF tensile strength decreased with relative humidity during preparation of specimens. CF tensile modulus increased with speed of loading increasing during test.