Size-dependent piezoelectric coefficient d 33 (D) (D shows particle diameter) for small ferroelectrics is modeled in this paper. It is found that taking account of size effect is essential in understanding piezoelectric characteristics of nanoferroelectrics. The model prediction is in good agreement with the experimental results for PbTiO 3 nanoparticles, where d 33 (D) increases correspondingly as D decreases. In addition, the size effect of the dielectric susceptibility coefficient η 33 (D) is also predicted since η 33 (D) ∝ d 33 (D) is considered. Our model estimations for η 33 (D) function are consistent with other theoretical evidences.