In order to reduce the size error in processing of slender shaft, the mechanics model of slender shaft under the effect of radial force was established, as well as the turning thermal deformation model of tool. The error formula was deduced related to the parameters of slender shaft. A method of dimension error compensation on slender shaft was introduced, the error compensation method was combined with a formula of surface error, which was used in the slender shaft experiment of NC turning. The results show that the size of machining error is decreased significantly by the compensation method, and the processing precision of slender shaft is improved.