High entropy alloy (HEA) is a new class of metallic materials, whose design was used to follow a simple route by assuming random mixing of atoms and maximizing their configurational entropy of mixing. In this talk, we would present a new paradigm for the design of HEAs of single phase solid solutions. By taking into account the formation enthalpy and excessive entropy of mixing, arising from dense atomic packing and atomic size misfit, a criterion based on a single parameter φ was developed, showing that the larger is φ the more likely is to form a single-phase solid solution in HEAs. Complementary to the φ criterion, a geometric model was also developed to calculate the intrinsic residual strain in HEAs, which predicts the formation of a single-phase solid solution if the root-mean-square (R.M.S.) residual strain is less than 5%. Our criterion and model agree well with the experimental results.