Dissolved oxygen in the ultrapure water has been known to oxidize Si wafer. In EEM, ultrapure water is also employed as a carrier fluid of ultra-fine powders. Oxidization properties of Si(100) wafer surface under EEM setup were investigated in this work. An oxidization with the growth speed higher than several 10nm/h was observed. The distribution of the oxide film thickness seems to depend strongly on the flow pattern on the Si wafer surface. The high speed oxidization observed in this work was understood to concern both to the extent of shear flow rate on the Si wafer surface and to contents of not only dissolved oxygen but also OH^- ion in the ultrapure water.