In conventional theories and calculations of thermal deformation of cylindrical pieces in steady temperature field, thermal expansion coefficients, body form parameters and temperature are considered. They are approximate and linear, can only be applied in normal or low precision field. An equation for calculating thermal deformation of cylindrical pieces in steady temperature field was deduced based on the linear relation between linear expansion coefficients and bulk expansion coefficients. Results of the theoretical and experimental studies were presented, the experimental results also made it clear that the equation is more logical than the traditional method.