Field experiment for controlling cucumber downy mildew by spraying 10% dimethomorph water emulsion was carried out in 2007 to identify its controlling efficiency and the operational safety.The results showed that 10% Dimethomorph water emulsion has a better effect on controlling cucumber downy mildew.The effect achieved 82.79 to 88.77 percent,which was significantly better than the control Anke Pharmacy 50% WP（79.18%）.It was safe to growth of cucumber.