Abstract: The single fiber fragmentation test (SFFT) was used to study the interfacial shear strength (IFSS) of the high---performance organic fibers polybenzoxazole (PBO), aramid fiber Twaron and ultra high molecular weight polyethylene (UHMWPE) fiber/tough epoxy by statistics on the filament’s saturated break points and analysis of the breakpoint morphology. The results were compared with the interlaminar shear strength (ILSS)　of unidirectional composites in the interply shear test. By means of XPS and SEM, the effect of the filament’s physical and chemical surface characteristics on the IFSS of fiber/matrix was also studied. The results show that the IFSS of Twaron/epoxy is higher than that of PBO/epoxy, and the strength of UHMWPE/epoxy is too low to be measured using SFFT. And both the IFSS and the ILSS are consistent for all three kinds of organic fibers/epoxy. Fibers with higher surface chemical activity have higher IFSS.