Based on the chemical equilibrium equation

2NO(g) + O₂(g) <--> 2NO₂(g) <--> N₂O₄(g) (1)

this paper deals with the possible gases existing in the reaction system and the change of the total volume of gases after the mixture of two volumes of NO and one volume of O₂ in a closed container under normal pressure (1 atm) with the changes in temperature. Meanwhile, we have obtained an equation for the calculation of the NO₂ polymerization equilibrium constant Kp in the reaction system on the condition that the temperature is within the range of 294.2-413 K and pressure is 1 atm. Kp = [(2V-Vm)áVm]/[4(Vm-V)²] With a glass syringe as the reaction container, we use a water bath to maintain a constant temperature for the determination of the dimerization equilibrium constant of NO. The result of our determination tallies with the documented value.