Expanding with heat and contracting with cold are common physical phenomenon in the nature. Variation in temperature leads to corresponding variation in physical dimension of mechanical parts. The conventional theories and calculations of thermal deformation are approximate and linear, can only be applied in normal or low precision field. The shortages of conventional calculations are pointed out. According to the research results of many years, a theory is given that the thermal deformation error of mechanical parts doesn't follow the conventional linear formula, it relates to all physical dimension of the mechanical part, and the deformation can be indicated by a nonlinear formula of physical dimensions. Studies on some common mechanical parts in precision technology have went on and the mathematical models have been set up, cylindrical piece, hollow piece and sphere are included. The experimental results also make it clear that these models are more logical than the traditional one. Because the sphere has only one physical dimension, so the thermal expansion coefficient can be achieved by detecting the thermal deformation of the sphere, and this method is more logical than the traditional method.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.