Ren, Y., Zhu, J., Hussain, N., Ma, S., Ye, G., Zhang, D. and Hua, S. 2014. Seedling age and quality upon transplanting affect seed yield of canola (Brassica napus L.). Can. J. Plant Sci. 94: 1461-1469. Seedling quality is an essential indicator for seed yield in canola, which is affected by many factors, including seedling age. Two field experiments were conducted to compare canola seedling quality and seed yield on 30-, 35-, 40-, 45-, 50-, 55-, and 60-d-old seedlings in 2011 and 2012. The relationship between seedling quality traits and seed yield of different seedling ages was also analyzed. Results revealed that the highest seed yield obtained from 40-d-old seedlings was attributed to more branches and siliques per plant. The negative effect of young seedlings (30-d-old) on seed yield was greater than that of old seedlings (60-d-old). The reduction rates in seed yield on the 30- and 60-d-old seedlings were 25.7 and 18.2%, respectively, compared with the 40-d-old seedlings. Increased root neck diameter, green leaf number, shoot, and root dry matter was the case on 40-d-old seedling transplanted plants compared with other ages. However, the increase was larger in the old seedlings than in the young seedlings. On average, the shoot and root dry weights of the 30-d-old seedlings were 1.9 and 1.7% of those in the 60-d-old seedlings. However, correlation analysis revealed that the seedlings with the highest shoot and root dry matter did not necessarily obtain the highest seed yield. Factor analysis suggested that the effects of root neck diameter and green leaf number on seed yield were more pronounced than those of shoot and root dry matter. Therefore, high seed yield in canola could be defined in terms of optimum green leaf numbers and branches per plant.