Abstarct: Iran has arid and semi-arid climate, and water shortage limits the growth and development of plants. Therefore, it is necessary to evaluate the response of plants to various levels of soil water contant. Accordingly, during the 2006-2007, a field experiment was conducted in research-farm of Shahrood University of Technology to study the effects of drought stress on yield, yield components, oil and protein content of winter canola, and to quantify the optimum irrigation intervals. The treatment levels were irrigation at 65% (control), 55%, 45%, 35% of Field Capacity (F.C.). The experimental design was randomized complete bloc design (RCBD) with 4 replications per treatment. The time of irrigation was determined, using the Flask-Method (a quick method of soil moisture measurment) by which the soil water content is rapidly defined. In this study, some attributes, including plant height, the length of branch, number of branches, harvest index, grain yield, straw yield, total dry matter, number of pod per plant, number of grain per plant, 1000- grains weight, oil and protein percent were measured. The results revealed that harvest index and 1000- grains weight are not affected by drought stress. Compared to control, the second level of drought stress (i.e. irrigation at 55% of F.C.) appeared to decrease no. of pod per plant (22%), plant height (24%) and total dry matter (18%). Although this decrease was not consistent with increasing the stress intensity, the lowest value of these attributes was obtained for fourth level of stress. The value of oil percent, straw yield, the length of branch, and no. of branch tended to be the same for control and second level of stress, but the lowest for fourth level of stress (the value of named attributes were 8%, 36%, 42% and 39% lower for fourth level of stress than for control, respectively). The protein content was increased with increasing the drought level (about 9% higher for fourth level, as compared to control). The drought stress imposed a decreasing effect on NAR, CGR, RGR, LAI and TDM. The irrigation interval for the control (i.e. first level) appeared to be 7-9 days (i.e. 7 days for warmer conditions, and 9 days for cooler conditions). For other levels, i.e. second, third and fourth levels, it was 8-11, 11-12 and 13-15 days, respectively. baxsed on above mentioned issues, and considering the amount of grain yield and oil content, it seems that irrigation interval 8-11 days (irrigation at 55% of F.C.) could be recommendable. It should be mentioned that although the highest protein percent was obtained for fourth level of drought, the amount of protein per hectare was the lowest for this level.