This paper proposes an optimal design of the thermal-actuated, piezoresistive-sensed resonator fabricated by a foundry-provided CMOS-MEMS process. The optimal design is achieved both by quantitatively comparing the mechanical properties of different composite films as well as by deriving an analytical model for determining the device dimensions. The analytical model includes a stress model of an asymmetric mechanical structure and a piezoresistivity model of the heavily doped, n-type polysilicon film. The analytical model predicts that the optimal length of the displacement sensor is 200 μm when the thermal actuator is 200 μm in length and the absorption plate is 100 μm in length. Additionally, the model predicts the resistivity of the polysilicon film of (6.8 ± 2.2) mΩ cm and the gauge factor of (6.8 ± 2.9) when the grain size is (250 ± 100) nm. Experimental results agree well with simulation results. Experimental data show that the resonant frequency of the device is 80.06 kHz and shifts to 79.8 kHz when a brick of Pt mass is deposited on the resonator. The mass of the Pt estimated from the frequency shift is 4.5419 × 10 -12 kg, while estimated from the measured dimension is 4.4204 × 10 -12 kg. Sensitivity of the resonant sensor is calculated to be 1.8 × 10 2 Hz ng -1 . Experimental results further show that the polysilicon film used in the experiments has a grain size of (241 ± 105) nm, an average gauge factor of 5.56 and average resistivity of 5.5 mΩ cm.