A microwave excited atmospheric pressure plasma source was manufactured and a cleaning experiment involving a stainless steel plate contaminated with organic oil was conducted, while considering the radical density in the Ar, Ar-O(2), and Ar-H(2)O plasma. The most notable cleaning effect was confirmed when the plasma contained water molecules. While the existence of a unique spatial distribution of OH radical was demonstrated, the relevance of the cleaning effect and radical density was confirmed. The radical density distribution was examined by numerical computation using the experimentally obtained plasma parameter. (C) 2009 Elsevier B.V. All rights reserved.