The H+-sensing behaviors of barium strontium titanate (BST) ultrathin films are reported. The sputtering technique was used to prepare the Ba0.7Sr0.3TiO3 ferroelectric structure. To obtain better electrical characteristics for the BST membrane, we characterized electrical properties of the BST/SiO2 thin film for the gate oxides in the metal-oxide-semiconductor devices under different deposition times of 10, 20, and 30 min using high frequency capacitance-voltage and current-voltage measurements. The pH-sensing properties of the stack BST/SiO2 layers were also investigated using capacitance-voltage measurements on the electrolyte/insulator/semiconductor. The results show that BST ultrathin film (< 30 nm) still exhibits linear pH sensitivities of approximately 50-58 mV/pH ranging from pH 1 to 11. Besides, the drift rate and temperature dependence of the measured pH sensitivity has also been represented.