Silicon Nitride integrated photonic circuits have drawn much attention owing to its ultra-low loss and large Kerr nonlinearity. However, the lack of Pockels effect makes it difficult to be modulated clectro-optically, which posts a major challenge for the further development of Si3N4 circuits with advanced functions. The widely adopted thermo-optical tuning suffers from large power consumption and restricted speed (similar to 1 kHz). In this study, microwave frequency modulation (up to 9 GHz) of Si3N4 ring resonator is achieved by exciting bulk acoustic waves piezoelectrically, which modulates the microring via stress-optical effect. The acoustic waves are confined tightly in a released SiO2 thin film which enhances the acoustic energy density and thus modulation efficiency.