The Downy mildew of onion, caused by Peronospora destructor Berk, generates a lot of economic damage to onions annually. Fungicides are the main components of plant diseases management. The aim of this study was to determine the efficacy of equalizer to control of onion downy mildew. Experiments were carried out in a randomized complete block design with six treatments and three replications in 2017 in Khuzestan and Hormozgan provinces. treatments consisted of equalizer (0.2, 0.3 and 0.4 kg /h, Infinito 2L /h, water spray and control without spraying). After observing the first signs of the disease, the treatments were conducted and continued every 10 days until the control treatment contamination reached as high as possible. Evaluation of experimental plots was conducted before each spray by selecting 10 plants from two middle rows and disease severity index (DSI) for each treatment was determined using scoring system of 1-9 and Kofoet A and Fischer formula. Analyses of variance revealed that there was significant diference among treatments at 1% level. But between treatments and location there was not significant difference. Also the result showed significant difference between location and treatment × location. The means comparison showed that 0.4kg/h Equalizer treatment creates statistically higest efficiency in Khuzestan and Hormozgan provinces(88/87 and 91/85%) respectivley. The other treatments including 0/2kg/h, 0/3kg /h of Equalizer and Infinito(2L/h) with 55/63,62.40 and 60.15 efficiency in Khuzestan and also 14/81, 76.29 and 67/40% efficiency in Hormozgan province were located in next statistcally groups. So the results of this study suggested 0.4kg/h Equalizer as appropriate rate in comparison to Infinito to control of Downy mildew of onion.