A method of machinning the inner shape of the dental crown was reported.The machine was controlled numerically and the data was based on the measured points of the prepared tooth with the 3D-point digitizer.Generally, the inner site of the dental crown has been manufactured with a NC(Numerical Controlled)machine using detailed measured data.However, the expression of the prepared tooth surface was not with the assemblies of the large amounts of measured points but with the interpolation of small amounts of measured points.Similarly, for the tool pathway, offset-surface data was calculated with the aid of the spline function.At first, for the measured points, the offset points had to be calculated, and these offset points were interpolated with the spline function.Thus the offset surface data was obtained.Calculation was easier with fewer original measured points.In this method numerosu calculations were not required to determine the offset points individually.Here, offset points were determined by the vector products of the two tangent vectors at each measured point.The directions of the offset vectors were opposite of those for the outside of the crown, because one surface value expressed both concavity and convexitywith the difference.For this conversion, the Z values of the measured points had to be changed to negative.To prevent the production of a mirror image, the Y values in addition to Z, were also transformed.