Graphene sheets produced by chemical vapor deposition (CVD) are polycrystalline and the presence of grain boundaries (GBs) alter their mechanical properties relative to single-crystal graphene. In this study, we have performed a series of molecular dynamics simulations using REBO2+S potential in order to develop a failure criterion for infinite polycrystalline graphene sheets under biaxial tension. We have studied the effect of temperature on mechanical properties of polycrystalline graphene for both uniaxial and biaxial loading conditions. The normal stresses are normalized with respect to the corresponding uniaxial ultimate strength values and the normalized stresses are used to define the failure envelope of polycrystalline graphene. Our study suggests that a bilinear failure envelope or a circular failure envelope can be used to represent with reasonable accuracy the tensile strength of polycrystalline graphene under biaxial loading at different temperatures.