A single functional leaf is usually sampled to evaluate the growth and photosynthetic assimilation of crops. However, there is large variation between leaves in winter oilseed rape (Brassica napus L.) at seedling stage. In this study, the morphological and physiological characteristics of various functional leaves were compared with characteristics of the whole plant at seedling stage for 2 years by using the oilseed rape cultivars Huaza 9 and Huaza 62 as plant material. The aim of this study was to identify a leaf that can represent the whole plant for assimilate accumulation characteristics at the seedling stage of the crop. The results showed that the photosynthetic rate and the contents of non-structural carbohydrates, nutrients and soluble proteins in fourth and fifth leaf of a plant were most closely related to those of the whole plant. The area and dry matter (DM) of all functional leaves were well correlated with those of whole plant, with the fifth leaf having the highest correlations. It is therefore recommended that the fifth leaf is most suitable to represent the whole plant for evaluation of growth and assimilate accumulation for winter oilseed rape at the seedling stage. The following regression equations for whole plant (y) and fifth leaf (x, dry matter or length × width) can be used to predict (1) DM accumulation (g) and (2) total leaf area (cm²) of the whole plant: (1) y = 3.32x   1.51 (R² = 0.88, P < 0.001); (2) y = 1.24x   222.69 (R² = 0.67, P < 0.001).