To study the stress triaxiality in the neck region of the circumferentially notched tension bars, the distribution of strain and stress are calculated analytically in toroidal coordinates.We suppose the coordinate line of toroidal coordinates form principal lines of stress and strain rate tensor with the plastic zone and during the process the shape of the notch is a circle and coincides with a Ψ coordinate line.Numerical examples reveal the relationship between the specimen geometry and the maximum value of stress triaxiality at the neck.The maximum value of stress triaxiality increases as the radius of the notch decreases.The result provides theoretical basis for investigating the relation between stress triaxiality and fracture types by uniaxial tension experiment of different circumferentially notched bars.