A field experiment was conducted to study the effects of inoculating earthworm (Metaphire guillelmi) on the yield components, seed yield, and seed oil content of winter oilseed rape (Brassica napus L. cv. Zhongshuang 9). Inoculating earthworm increased the primary branch numbers per plant, main raceme pod numbers per plant, seed numbers per pod, and 1000-seeds weight, but the effect was not significant. However, comparing with the control, inoculating earthworm increased the pod number per plant, seed yield per plant, and seed yield of whole plot significantly, with the increment being 36.7%, 46.5%, and 29.7%, respectively, which could be related to the promotion effect of earthworm on the plant growth and its nitrogen uptake at vegetative growth stage. After the inoculation with earthworm, the seed oil content somewhat decreased, but, owing to the significant increase of seed yield under the effect of earthworm, both the oil production per plant and the oil production of whole plot increased significantly by 37.4% and 21.0%, respectively, compared with the control.