An experiment was carried out on the basis of RCB design in 2008, to assess the effects of seed aging on field performance of winter oilseed rape. Seeds of winter oilseed rape (cv. Talayeh) were divided into three sub-samples. A sub-sample was kept as control or high vigor seed lot (V1). The two other sub-samples with about 15% moisture content were artificially aged at 40°C for 9 and 12 days (V2 and V3, respectively). So, three seed lots with different levels of vigor were provided. These seeds were then sown in a sandy loam soil and the performance of the seed lots was evaluated. The results showed that the mean emergence time and days to flowering significantly increased with increasing seed aging, but mean emergence percentage and winter survival decreased as seed deterioration increased. The number of grains per plant, 100 grain weight and grain yield per plant were not statistically affected by seed aging. It seems that early emergence of seedlings from high vigor seeds was compensated by efficient use of environmental resources by individual plants from aged seeds, due to poor stand establishment. In general, it was concluded that seed aging can reduce crop yield indirectly, through decreasing the rate and percentage of seedling emergence and winter survival. Therefore, it is necessary to produce and cultivate high vigor seeds of oilseed rape, in order to ensure satisfactory yield achievement in the field.