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Nano-sized SrTiO3-based oxygen sensors were fabricated from synthesized SrTiO3 and commercial SrTiO3 using the high-energy ball milling and the thick-film screen-printing techniques. The particle sizes, microstructural properties, oxygen-sensing properties, and humidity effects of the synthesized nano-sized SrTiO3-based oxygen sensors were characterized using X-ray diffraction (XRD), transmission electron microscope, scanning electron microscope (SEM), and gas sensing measurements. Experimental results showed that the particle size of the powders was milled down to be around 27 nm. The effect of different annealing temperatures (400°C, 500°C, 600°C, 700°C, and 800°C) on the gas sensing properties of the synthesized SrTiO3 sensor from nitrogen to 20% oxygen was characterized. The commercial SrTiO3 devices annealed at 400°C, both with 0-h and 120-h milling time, were used for comparison. The optimal relative resistance (Rnitrogen/R20%oxygen) value of 6.35 is obtained for the synthesized SrTiO3 sample annealed at 400°C and operating at 40°C. This operating temperature is much lower than that of conventional metal oxide semiconducting oxygen gas sensors (300°C-500°C) and SrTiO3 oxygen gas sensors (>700°C). The response and recovery times are 1.6 and 5 min, respectively. The detected range is 1-20% oxygen. The impedance of the synthesized SrTiO3 sensor with annealing at 400°C and operating at 40°C (from 1 mHz to 10 MHz) in 20% oxygen ambient was found to be independent of the relative humidity (dry, 20% RH, 80% RH, near 100% RH).