Ethaboxam is a fungicide controlling plant diseases caused by Oomycetes. Efforts were made to improve its fungicidal activity applying formulation technology. Fungicidal activity of ethaboxam against cucumber downy mildew caused by Pseudoperonospora cubensis was improved by incorporating polyoxyethylene cetyl ether (PCE) in a wettable powder formulation. It was found that the optimum combination ratio of PCE and ethaboxam was 3:1, and a tank-mix of 150 μg/ml of ethaboxam and 450 μg/ml of PCE would be as good as the standard 25% WP formulation diluted to 250 μg/ml ethaboxam without PCE in controlling cucumber downy mildew. Based on this results, a wettable powder (WP) co-formulation containing 15% of ethaboxam and 45% of PCE was developed in this study, and tested for its performance in the fields. This co-formulation showed significant improvement in persistence of fungicidal activity and curative efficacy of ethaboxam against cucumber downy mildew. The improved control efficacy was also confirmed for control of grape downy mildew caused by Plasmopara viticola and potato late blight caused by Phytophthora infestans in the field tests.