A material testing apparatus for measuring the biaxial deformation behavior of a polymer tube is developed to quantitatively evaluate the deformation behavior of polymeric materials. The testing apparatus can apply axial force and internal pressure to a tubular specimen. A non contact strain measurement system is also developed, and the biaxial strain components and the radius of curvature in the axial direction of the specimen can be continuously measured to perform control stress paths. Polyethylene tubes with an outer diameter of 17 mm and a thickness of 2 mm are used as test specimens, that are subjected to linear stress paths with stress ratios of σΦ:σθ=1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4, and 0:1, where σΦandσθ are the axial and circumferential stress components applied to the center of the bulging specimen, respectively. Biaxial stress-strain curves are successfully measured for every linear stress path at a nearly constant logarithmic strain rate of 1×10-3 s-1. A maximum equivalent strain of up to 0.71 is achieved for a specimen with a stress path ofσΦ:σθ=2:1. The contours of equal work plotted in the principal stress space show a significant anisotropy of the test material.