The theoretical model of the size dependent piezoelectric coefficient d33(D) of the ferroelectric PbTiO3 nanoparticles was established in this paper.It is found that taking account of size effect is essential in understanding piezoelectric characteristics of nanoferroelectrics. The d33(D) increases with D decreases and the model prediction is in good agreement with the experimental results for PbTiO3 nanoparticles.In addition,the size effect of the dielectric susceptibility coefficient η33(D) is also predicted supposing that η33(D) is proportional to d33(D).Our model estimations for η33(D) function are consistent with other theoretical evidences.