In this work, 5-20 at% gas-accessible WO₃-NiO hollow nanoflowers were synthesized through a one-step hydrothermal route and used to fabricate metal oxide semiconductor (MOS) based gas sensor. The gas-accessible WO₃-NiO hollow nanostructures showed much larger BET surface areas (168.0-203.8 m²g^-1) than that of the pure NiO (45.9 m²g^-1). In the comprehensive gas sensing test, the gas device based on 10 at% WO₃-NiO hollow microspheres exhibited the best xylene sensing performance, showing ultrahigh xylene sensitivity (354.7-50 ppm) with short response-recovery times within 1 min. (51 and 57 s respectively) and ultralow detection limit (1.5-50 ppb xylene). Additionally, the 10 at% WO3-NiO based sensor also showed superior xylene selectivity against other interfering gases in a wide temperature range (250-350 degrees C). Especially at the optimal 300℃, the 50-ppm xylene sensitivity was 8.1 and 10.3 times higher than that of 50-ppm representative acetone (S-xylene/S-acetone = 8.1) and ethanol (S-xylene/S-ethanol = 10.3) gases, respectively. The mechanisms for the excellent xylene sensing performance were also discussed.