This paper demonstrates the estimation of vibrational temperatures of N₂ and CO₂ in low-pressure plasmas by threshold ionization mass spectrometry. The principle for the estimation is the decrease in the ionization potential by the vibrational excitation. We observed that the threshold ionization curves of N₂ and CO₂, which were measured using a quadrupole mass spectrometer with an energy-variable electron beam, shifted toward the low-energy side, when they were sampled from the plasmas. We constructed a model which assumed a Boltzmann distribution for the population densities of vibrational excited states and the same cross sections of electron impact ionization for vibrational excited states except the shifts of the threshold energies. The vibrational temperatures were estimated by fitting the experimental threshold ionization curves with the model.