To study the effects of sowing dates on seed germination and on seed vigor of 12 spring rapeseed cultivars, an experiment was performed in years 2005-2006. During the first year, 12 cultivars of spring rapeseed were planted in the Research Field of Seed and Plant Improvement Institute, Karaj, Iran. During the second year, standard germination test was conducted to study the performance of the seeds grown during the first year of the experiment. A factorial experiment in randomized complete block design was employed. According to the results, the seeds grown during the two seasons were significantly different from the view point of seed weight, germination, length of radicle and plumule, seedling vigor index, number of normal and abnormal seedlings, mean daily germination as well as daily germination index. In addition, the effect of cultivar was significant on seed weight, germination, length of radicle, plumule and seedlings, seedling vigor index, number of normal and abnormal seedling, mean daily germination as well as daily germination index. The seeds of autumnal cultivation were better off from the view point of seedling vigor as well as other important characteristics of seed vigor. Among the cultivars, RG405/02 was better than the others due to its high seed weight. A reduction of seed weight during germination could be used as a valuable index to determine seed vigor.