Nanomodified Portland cement paste with a water/cement ratio of 0.4 was prepared with the addition of TiO2 nanoparticles at 0.1, 0.5, 1.0, and 1.5% by mass of cement. The flexural strengths of the prepared cement-based composites were tested, and the fracture surfaces were observed by scanning electron microscopy (SEM). The flexural strength of the nanomodified TiO2 Portland cement paste reached the highest value with a dosage of 1.0 mass %. The SEM observation shows that admixing of the TiO2 nanoparticles largely decreased the quantity of internal microcracks in the cement paste. A new type of needle-shaped hydration product was observed, and its potential growth mechanism was proposed. Atomic force microscopy was introduced to observe the microstructure of nanomodified Portland cement paste, and the results show that the nanoroughness of the hardened cement pastes with admixed TiO2 nanoparticles was much lower than that without the TiO2 nanoparticle addition. Coupled with X-ray diffraction data, the morphological information obtained at the micrometer and nanometer scales shed light on the role of TiO2 nanoparticles in the cement-based composite.