The lateral spread of implanted P+ions and the shape of the mask window have been observed simultaneously using the technique of staining the cleaved surface and scanning electron microscopy for the Si samples with the SiO2mask window with a tapered edge. The mask edge with a gradient of 45° or 78° to the Si surface and the implantedn‐type region with a carrier concentration higher than 2×1017/cm3are observed in the same photograph. The observed maximum lateral spread when the gradient of the mask edge is 45° is about 1.6 times larger than that when the gradient is 78°. The calculated results of the lateral spread agree relatively well with the experimental data although the precise analysis based on the definite basis is necessary.