Electrochemical water splitting has been intensively pursued as a promising approach to produce clean and sustainable hydrogen fuel. However, the lack of low-cost and high-performance electrocatalysts for the hydrogen evolution reaction (HER) hinders the large-scale application. Herein, we have rationally designed and synthesized 3D self-assembly architectures assembled from ultrafine MoC nanoparticles (0D) uniformly embedded within N-doped carbon nanosheets (2D) for the HER via a simple protocol. The well-organized 3D nanostructures are composed of very small MoC nanocrystallites (<2 nm) and free-stretching conductive carbon nanosheets with high specific surface areas and abundant mesopores, which can expose more active sites and facilitate electron/ion transport pathways. Based on the merits of the composition and configuration, the resultant hierarchical 3D self-assembly architectures exhibit remarkable electrocatalytic performance and stability for the HER.