SnO₂ nanoparticles are successfully synthesized using a facile solvothermal method without any templates or surfactants at a relatively low temperature of 100 °C following an annealing process. As-synthesized SnO₂ samples exhibit a uniform nanoparticle morphology with a diameter around 9 nm. The gas sensing properties of the as-fabricated sensor based on SnO₂ nanoparticles are carefully investigated and the results exhibit a high response, fast response/recovery speeds, good sensitivity, a low detection limit and superior repeatability for the volatile organic compound formaldehyde at a relatively low operating temperature. The formaldehyde sensing mechanism is discussed in detail and a possible growth mechanism is proposed.