This paper is concerned with the empirical flow stress constitutive equation of steel sheets for an auto-body with the variation of temperature and strain rate. Dynamic tensile tests are carried out with the steel sheets at various temperatures. Both the strain rate and the temperature sensitivity of the flow stress are evaluated at the intermediate strain rate. In order to describe the thermo-mechanical behavior of the steel sheet at the intermediate strain rates accurately, a hardening equation is newly suggested by modifying the well-known Khan–Huang model. The suggested model gives good correlation with experimental results at various intermediate strain rates and temperatures.