In the progress of semiconductor technology, the diameters of Si substrates are increasing and the flatness accuracy is being raised. However, the evaluation method has not been accomolished. The sequential-two-points method, which was originally developed to evaluate straightness error motion of the tool as well as error profile of machined parts in machine tool, is applied to evaluate surface form of the flatness on the substrates. Forms along diameters and periphery are measured as a step to construct form of flatness on polar coordinate. Resolution of the capacitor type sensors are calibrated with order of nm, so that the measurable accuracy, of the method is assured and the straightness error from of 0.3 μm along diameter have been obtained, on the other hand which can be easily deformed by boundary conditions of support.