Abstract Nanoscale impact can bring big changes in micro-meso-macroscale behaviors of the composites. The addition of nano fillers makes cementitious materials stronger, more durable and multifunctional/smart. This paper aims at investigating the underlying mechanism for understanding and controlling the nano-engineered cementitious composites. The nano-core effect is proposed through integrating core-effect with nano effect, and is proved by experimental evidences for the cementitious composites with different nano fillers. The nano-core effect is closely relative to the intrinsic properties of nano fillers, composition and processing of the cementitious composites. The behaviors of the nano-engineered cementitious composites are governed by nano-core effect zone, i.e. nano-core-shell element. It is therefore concluded that the nano-core effect is fundamental for design, fabrication and application of the nano-engineered cementitious composites.