Neat and Pd-loaded WO3 sensors were fabricated from lamellar-structured nanoparticles. The sensing mechanism of hydrogen was investigated. Oxygen adsorption properties were studied through the resistive responses and TPD measurements. It was observed that oxygen adsorption was significantly enhanced by Pd loading from the viewpoint of p-n junction for PdO-WO3. In the presence of H-2, the resistive response to oxygen for Pd-loaded WO3 was obviously different from that of neat WO3, which demonstrated a relatively weak response to oxygen. It was concluded that Pd loading not only promoted the sensor response but also made the sensing mechanism different from that of neat WO3. It was proposed that with Pd loading, the surface lattice oxygen near Pd sites was involved for the sensing process of H-2.