Xylene is a harmful and hazardous volatile organic compound (VOC) indoors, thus selective and sensitive detection for subppm-level xylene is crucial, however a remained challenge. In this work, p-NiO/p-NiCr₂O₄nanocomposites were successfully synthesized through a simple hydrothermal route and used as sensing materials. In the comparative gas sensing test, the sensor fabricated from NiO/NiCr₂O₄(Cr/Ni=25 at%) nanocomposite exhibited the highest response (66.2-100 ppm) to xylene, which was 37.2 times higher than that of the pure NiO sensor. Moreover, the NiO/NiCr₂O₄nanocomposite gas sensor showed not only superior xylene selectivity with low cross-responses to interfering gases such as ethanol (S-xylene/S-ethanol = 11.8) and acetone (S-xylene/S-acetone = 10.2) but also ppb-level detection limit (1.2-50 ppb xylene) at 225℃. The synergistic catalytic effect between NiO and NiCr₂O₄, optimized structural parameters and marked resistive variation due to the formation of nanoscale p-p heterojunctions were regarded as the main reasons for the ultrasensitive and selective xylene detection.