Field trials of flax plants were sown during two successive growing seasons 2006 and 2007at Gemiza Agriculture Research Station, Gharbiya governorate, Egypt. The effects of relative humidity (RH) and temperature on flax powdery mildew (Oidium lini Sikoric) were studied in controlled environments to define conditions that affect disease development in flax. In this study, four cultivars; Sakha2, Giza8, Liflora and Escalina were evaluated for disease development. Two different fungicides; Bayfidan, Tobas 100-EC and two novel antifungal compounds; Bion and Tannic acid were foliar sprayed at three intervals to control powdery mildew. Data showed that the germination of conidia and symptoms appearance at a constant temperature at 25˚C failed to occur at 0% RH but by increasing the RH to 50, 60 and 75% infection process occurred by conidia and symptoms appeared. The highest level of conidia germination and infection establishments expressed as high diseases incidence percentage was observed at 25˚C and 75-100% RH.  Disease control with fungicides and antifungal compounds showed significant differences among all treatments sprayed with Tobas 100-EC, Bayfidan, Bion and Tannic acid compared with the control. Significant effects were observed on yield, yield components, technological characters for fiber and chemical character of seed oil.