Oilseed rape cultivated surfaces sharply decreased worldwide recently, mainly due to poor plant establishment conditions. Indeed, seedlings are affected during autumnal or spring growth by various abiotic and biotic stresses and thus impacting yield potential. Improving plant establishment using vigorous varieties is a promising way, especially in low-input and agroecological systems. The ability of the plant to develop rapidly, from its emergence to the 4-5 leaf stage, is called early vigour. It is an integrative and a complex trait, often approximated by leaf area or total biomass but its underlying processes remain unclear even though they are potential selection traits. Therefore, this study aims to identify those processes and to describe their associated genetic diversity. A population of 217 genetically diverse B. napus lines was phenotyped for early vigour. It included 118 winter (WOSR) and 86 spring oilseed rape (SOSR), 11 winter fodder and 2 rutabagas, with high (++) and low (00) erucic acid and glucosinolate seed content. Experiments were conducted in a tunnel in Le Rheu (France) from sowing to the 4-5 leaf stage. Early vigour was estimated by the leaf area and the plant total biomass at harvest. Leaf cover dynamic was estimated each day by image analysis. Traits related to the following biological processes were assessed: biomass production (shoot and root biomass, specific carbon assimilation (SCA)), plant development (leaf area, leaf surface to shoot nitrogen ratio (LASNQ)), leaf growth (area under the curve (AUC) of leaf cover dynamics, phyllochron) and nitrogen content. PCA and random forest analyses were performed to investigate the relationships and trade-offs between these processes and their contribution to early vigour. Groups of genotypes with contrasting patterns were assessed. The main processes contributing to early vigour were the specific AUC, SCA and LASNQ. For these traits, genetic diversity was observed and different patterns were identified between SOSR (++ and 00) and WOSR (++ and 00). As a whole, SOSR and specifically some SOSR++ developed faster (shorter phyllochrons, higher AUC) and produced equivalent or greater total biomass and leaf area than WOSR. WOSR00 presented the worst AUC values. Most of the top vigorous lines were SOSR ++. When looking more particularly at the differences between phenotype correlation matrices of WOSR and SOSR, it was observed that SCA was positively correlated to AUC and biomass for SOSR and inversely for WOSR. On the other hand, LASNQ was positively correlated to AUC and biomass for WOSR and inversely for SOSR. These results bring more knowledge on the processes involved in early vigour and their related genetic diversity. Potential selection traits were identified for early vigour. The germplasm characterization showed that multiple SOSR ++ lines were shown to be more vigorous than WOSR, especially WOSR00. This highlights their value to develop breeding program targeting plant vigour as a mean to further improve plant establishment. The next step will be to identify the genomic regions involved in vigour-related traits and decipher the genetic architecture of early vigour.