To decrease the straightness errors and improve the accuracy of machine tools, a spatial straightness error modeling which is based on B-Spline curves is presented. Experiments are carried out on a machine tool to obtain six serials of straightness errors by laser interferometer and a space straightness error modeling is obtained by applying the B-Spline method. Compensation is implemented by applying the error compensation system which utilizes the external mechanical origin offset function of Fanuc to impute the compensation data into CNC. Compared with the polynomial model and the interpolation model, the B-Spline model is proved to be of high accuracy. In addition, the straightness errors of the machine tool are deceased by more than 90% compared with no compensation, the machine accuracy is efficiently improved.