To study the efficacy of new fungicide, Difenoconazole + Cyflufenamid (Sidly-Top® DC 140) in controlling cucumber powdery mildew disease caused by Golovinomyces cichoracearum, experiments were carried out in Alborz, Tehran, and Khuzestan provinces in a Randomized Complete Block Design(RCBD) with 6 treatments and 4 replications under glasshouse conditions. Treatments included 3 recommended doseses by Syngenta including 0.08, 0.09 and 0.1% of Sidly-Top®, tetraconazol (Domark® EC 10%) 0.04%, bicarbonate potassium (Kaliban® SL 85%) 0.5% along with control without any spraying. Foliar applications of fungicides started once after early symptoms appearance and followed up at 5-10 days intervals. The results were expressed before each spraying and 14 days after last spraying as the disease severity index (DSI) using the Horsfall and Barrat scale. Results revealed that doses 0.08, 0.09, and 0.1 % of Sidly-Top® and Domark 0.04% had the same effects at 5-10 days interval sprayings. The 4th and 5th assessments revealed that the SidlyTop 0.09% was the best choice for recommendation at 10-14 days interval sprayings. The comparison of efficacy of treatments on AUDPC showed that doses 0.08, 0.09, and 0.1 % of Sidly-Top® have reduced the percentage of Area Under the Disease Progress Curve by 79.8, 87.2, and 90.2 %, respectively compared to control with no spraying. Meanwhile, the fungicide Domark®at the rate of 0.04% resulted in the efficacy of 91.2% in comparison with control. The fungicide Kaliban® also showed the least amount of efficacy (53.1%) with highest daily progress of disease (18.7 %) compared to control. Based on the results gained from this research, the fungicide Sidly-Top® may be recommended to be used at 0.08% for 5 to 7-days intervals and at 0.09% for 10-14-days intervals sprayings.