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Optimum leaf area index (LAI) is necessary to decrease in soil surface evaporation, to intercept maximum radiation and to increase in dry matter production and seed yield. In order to study the development of leaf area in canola (B. napus) and its relationship with seed yield, this experiment was conducted at agricultural research station of Gonbad during 2005-7. The experiment was a RCBD arranged in a split-plot in two conditions, i.e. supplemental irrigation and rain-fed. Five sowing dates (SD) (Nov. 6, Dec. 6, Jan. 4, Feb. 5 and Mar. 5) assigned as main plots and two cultivars (Hyola 401 and RGS003) as subplots. In all treatments, the number of leaves on main stem and LAI increased till the beginning of seed filling, and then decreased considerably due to sever shedding of leaves. LAI at the beginning of seed filling stage, varied from 6.1 in Nov. 6 to 3.1 at Mar. 5 SDs in supplemental irrigation conditions, and from 5.2 on Nov. 6 to 2.4 at Mar. 5 SDs in rain-fed conditions. The better early growth of Hyola401 compared to RGS003 cultivar caused that, in each year and conditions, LAI of Hyola401 was significantly more than RGS003 at the beginning of stem elongation and budding. The strong relationships between the number of leaves on main stem and LAI at different growth stages and seed yield, showed the positive effects of these traits on seed yield.