Abstract: The phenomena,taking place during stretching of the epoxy resin matrix con-taining a single carbon fiber treated with different methods ,are researched. During stretch-ing,it is found that obvious photoelastic phenomenon appears around the carbon fiber treatedwith plasma,no clear photoelastic phenomenon around untreated one and only weak lines ofphotoelasticity around grafted fiber. Based on mean fragment length of single carbon fibcr af-ter the matrix being stretched it is known that the bonding strength of interface(BSI)of thecarbon fiber treated with O_2 plasma is higher than the strength of epoxy matrix itself,so thatthe cracks of matrix will be generated along with the fiber in the vertical direction from thefiber axis. However ,the cracks in the PAA-grafted fiber/epoxy system can not be generatedalthough BSI between the PAA-grafted fiber and matrix is higher than the strength of resinitself as well.