We demonstrate a low temperature flattening method for the 200-mm-diameter (100) orientation silicon wafers. By annealing in ultra pure argon ambient at 850°C or above, atomically flat surfaces composed of atomic terraces and steps appear uniformly in the whole 200mm wafer. The width of atomic terrace changes with the off angle of wafer surface. It is found that with the off angle of 0.50° or below, only mono-atomic steps appear on the atomically flat surface, and the terrace widths are almost equal to the calculation values. Moreover, we have found using the vertical furnace the whole 200mm wafer surface can be atomically flattened in shorter time by increasing the argon gas flow rate or the annealing temperature. Furthermore, after annealing at 900°C or below, there is no slip-line defect in the whole wafer. This low temperature flattening method is very suitable to be applied in the LSI manufacturing.