A new method to improve the wet etching characteristics is described. The anisotropic wet-etching of (100) Si with megasonic wave has been studied in KOH solution. Etching characteristics of p-type (100) 6inch Si have been explored with and without megasonic irradiation. It has been observed that megasonic irradiation improves the characteristics of wet etching such as the etch rate, etch uniformity, surface roughness. The etching uniformity was less than ±1% on the whole wafer. The initial root-mean-squre roughness(Rrms) of single crystal silicon is 0.23nm . It has been reported that the roughnesses with magnetic stirring and ultrasonic agitation were 566nm and 66nm . But with megasonic irradiation, the Rrms of 1.7nm was achieved for the surface of 37µm depth. Wet etching of silicon with megasonic irradiation can maintain nearly the original surface roughness during etching. The results have verified that the megasonic irradiation is an effective way to improve the etching characteristics - the etch rate, etch uniformity and surface roughness.