Zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg xZn1-xO) are piezoelectric materials for high quality factor bulk acoustic wave (BAW) resonators operating at GHz frequencies. Mg xZn1-xO thin film BAW devices built on Si substrates are particularly attractive for integrating piezoelectric MgxZn 1-xO with the main stream semiconductor devices and circuits. In this paper, we report single-mode MgxZn1-xO based thin film resonators (TFRs) built on Si substrates. An acoustic mirror, composed of alternating quarter-wavelength silicon dioxide (SiO2) and tungsten (W) layers, is used to isolate the resonator from the Si substrate. High quality and well c-axis oriented MgxZn1-xO thin films are deposited on Si substrates using RF sputtering technology. X-ray diffraction (XRD) and field emission electron microscopy (FESEM) are used to characterize the MgxZn1-xO layers. The theoretical analysis of the TFR, based on the transmission line model, is presented. The BAW velocity and effective coupling coefficient of MgxZn1-xO can be tailored by varying the Mg composition in the films. The acoustic velocity increases with increasing Mg composition. The feasibility to use this structure to build ultra-high-sensitive mass BAW TFR sensor is analyzed. A mass sensitivity higher than 103 Hz cm2/ng is demonstrated.