Display omitted •Gas sensing properties of two types of Ni based metal-organic frameworks were examined.•Better gas sensing performance was related to the smaller pore size, higher surface area, and higher concentration of Ni2+ sites.•Selective detection of CO was achieved by the special interaction between open Ni sites and CO gas molecules. In this study, two different kinds of MOFs such as Ni-VNU-74-I and -II were examined for CO gas sensing. The Ni-VNU-74-I and -II showed different pore sizes and surface areas. The as-synthesized iso-reticular type Ni-VNU-74-I and -II sensors showed well crystallin chrysanthemums like morphology with great thermal stability, and surface areas (up to 2350 m2/g). Gas-sensing studies were performed systematically at different temperatures and towards different gases. The Ni-VNU-74-II gas sensor showed a stronger gas response (Ra/Rg = 1.7) to 50 ppm CO gas than to other gases at 200 °C because of the higher quadrupole moment of the CO gas molecules, leading to a strong interaction with a partial charge in the open metal Ni sites. In addition, the better performance of the Ni-VNU-74-II gas sensor compared to the Ni-VNU-74-I gas sensor (Ra/Rg for 50 ppm CO gas = 1.2) was related to the better interaction between the open metal sites (Ni) and CO molecules through a special structure of the Ni-VNU-74-II gas sensor with smaller pore sizes, higher surface area, and higher concentration of open metal Ni2+ sites. The sensing performance in this work indicates Ni-MOFs as the potential gas sensors for the detection of harmful gases in practical applications.