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We present a systematic study of uniaxial/biaxial stress effects on low-field mobility and on-current in high-kappa n/pFETs. It is found that mobility enhancement by strain in high-kappa FETs is smaller than SiO2 FETs in low effective field because of remote Coulomb scattering caused by fixed charges inside high-kappa films, while mobility enhancement by biaxial tensile strain in high-kappa nFETs is greater than SiO2 nFETs in high effective field due to weaker surface roughness scattering in high-kappa nFETs. In short-channel high-kappa nFETs, better on-current improvement by biaxial tensile strain than in SiO2 nFETs is achieved as a result of both higher mobility enhancement and weaker velocity saturation. The optimum stress design for high-kappa n/pFETs is also discussed, and it is concluded that the application of transverse tensile stress, in addition to conventional longitudinal stress, is essential for performance improvement of high-kappa n/pFETs.