Size effects on the lattice parameter and optical gap energy E g are investigated in SrTiO3 nanoparticles in the range of the average size D for 12–40nm. An anomalous lattice expansion is found in SrTiO3 nanoparticles, i.e., the lattice parameter a increases from 3.909Å at D = 40 nm to 3.920Å at D = 12 nm , which is in contrast with the lattice contraction in metal nanocrystals. We have ascribed the lattice expansion to the negative pressure effects due to the enhanced surface-defect dipole in the smaller particles. Such a model also explains change of the E g-value with particle size.