Different nanostructures of tungsten oxide (WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) can be hydrothermally synthesized by controlling the pH value through adding HCl solution. WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> has a square plate-like structure when the mixture solution has a pH value of 1.0. At this pH, WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanoplates are dispersed and did not formed clusters. Porous nickel oxide (NiO) spheres are also hydrothermally synthesized for decoration on the surface of WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanoplates. Holes on the NiO spheres increase the surface area of the material and enhance the sensing performances of the sensor. By combining these two materials, the sensor with p-n-n junction is sensitive to ethanol and has a high sensitivity of 155%. The response and recovery times are also fast with good sensitivity and selectivity. It has a high potential for applying to ethanol sensing in industrial and medical areas.