The interesting ZnO nanoparticles (NPs)-based sensor devices are fabricated on sapphire substrates to study the nitrogen oxide (NO 2 ) gas sensing performance. The postheat treatment process is not used to avoid the undesired agglomeration phenomenon. Different concentrations (0.1 and 0.01 wt%) of ZnO NPs are employed to produce the studied NO 2 gas sensors. Experimentally, studied devices demonstrate good NO 2 sensing responses \sim 100 (at 5 ppm NO 2 /air, 473 K). Especially, an extremely low sensing limit of 10 ppb NO 2 /air is obtained for the studied devices. This value is superior to those of previously reported results from other NO 2 sensors. The studied sensor devices also show the advantages of low cost, easy fabrication, and compact potentiality to integrate with other high-speed and optoelectronic devices. Therefore, the studied devices give the promise for high-performance NO 2 monitoring applications.