Abstract: Epoxy is the main matrix material for fiber-reinforced plastics(FRP) and is always in complicated stress state in FRP.The content of this paper may be summarized as follows:a) The relacionship betwcen streas and strain for epoxy cast has been established when it is in plane stress state and during linear elastic siage.b) Triaxial material tests have been made ott epoxy cast thitt-walled cylinderical samples when they arc in plane stress state where there arc six principal stress ratios(1:0,1:0.5,1:0.96,1:-1,1:0.38,1:-0.155).c) Analyzing theccommon reasons of the failure in the six types of tests,it may come to the conclusion that the maximum tensile stress theory can be applied to epoxy cast when its two principal stresses are both positive or when one is positive while the other is negative whose absolute value is less than that of the positive one.