The effect of grain size and phase compositions on piezoelectric coefficient of BaTi0.98Hf0.02O3 ceramics prepared at a series of sintering temperatures (1320, 1350, 1370, and 1400 degrees C) was studied. The results showed that the grain size of the ceramics is 0.9, 21.3, 21.6, and 37.2 mu m, respectively, and the corresponding phase compositions are the tetragonal-orthogonal, tetragonal-orthogonal-rhombohedral, tetragonal-orthogonal, and tetragonal-orthogonal-rhombohedral, while the piezoelectric coefficient is 475, 352, 258, and 327pC/N, i.e., it decreases first and then increases as the grain size goes up. The phase compositions and grain size of the ceramics are interrelated, and they co-affect the piezoelectric coefficient.