Abstract Effect of nano-TiO 2 on the mechanical properties of cement mortar was studied in this paper. Test results indicated that when cement was substituted by nano-TiO 2, the strength of cement mortar at early ages increased a lot and the fluidity and strength at evening ages decreased obviously. The change tendency of diffraction intensity of (0 0 1) crystal plane was entirely different with that of (1 0 1) crystal plane. It seemed that the main reason for the improvement of strength is the decrease and modification of orientation index for the nucleus function, not the increasing amount of hydration products. Results also indicated that it was useful to modify the fluidity and the strength at evening ages of the cement mortar with nano-TiO 2 by adding superplasticizer and slag powder into cement mortar.