This work presents a new method for detecting oxygen precipitates in Si wafers. The method utilizes a highly selective reactive ion etching process. When a Si wafer containing oxygen precipitates is etched under the condition of high selectivity of silicon over silicon oxide, needle-shaped silicon cones can be formed below the oxygen precipitates, because they are hardly etched. Using this method, the depth distribution of the oxygen precipitates having diameters of at least 15 nm has been successfully evaluated by evaluating the number and height of the cones. It was also demonstrated that this method is effective in detecting oxygen precipitates remaining in the denuded zone.