This work, using the first-principles theory, uses the Ni-decorated WSe 2 (Ni-WSe 2 ) monolayer as a novel gas sensing material upon CO and HCHO in the dry-type transformers in order to evaluate their operation status. Results indicate that the Ni atom can be stably adsorbed on the T W site of the pristine WSe 2 monolayer with the binding force of -4.33 eV. Via the gas adsorption analysis, it is found that the Ni-WSe 2 monolayer performs chemisorption upon CO and HCHO molecules, with adsorption energies of -2.27 and -1.37 eV, respectively. The analyses of the band structure and Frontier molecular orbital manifest the potential of the Ni-WSe 2 monolayer as a resistance-type gas sensor upon CO and HCHO, with sensing responses of 55.9 and 30.9% based on the band gap change and of 55.0 and 38.5% based on the energy gap change. The analysis of the density of state clearly shows the modified electronic property of the Ni-WSe 2 monolayer in gas adsorptions. On the other hand, the analysis of the work function (WF) reveals the limited possibility to explore the Ni-WSe 2 monolayer as a WF-based gas sensor for CO and HCHO detections. This work systemically studies the sensing potential of the Ni-WSe 2 monolayer upon two typical gas species in the dry-type transformers, which is meaningful to explore novel nanomaterial-based gas sensors to monitor the operation condition of electrical equipment.