Abstract: A critical tube diameter measurement system was established for C₂H₂-2.5O₂-Ar mixtures detonation. Initiation of detonation was accomplished via a high voltage spark energy source. The critical tube diameters under different initial pressures of C₂H₂-2.5O₂, C₂H₂-2.5O₂-50%Ar, C₂H₂-2.5O₂-70%Ar mixtures were measured in experiments. The results show that, with the increasing of argon dilution for the same tube diameter, the critical pressure, at which the planar detonation can emerge from a tube into unconfined space to form a spherical detonation, increases as well, that indicates the sensitivity of the mixtures detonation decreases. In addition, the relationship between critical tube diameter and cell size for those mixtures were investigated. Finally, the correlation between critical tube diameter and critical energy of direct initiation was analyzed. Good agreement was found by comparing the experiment data with Lee's surface energy model. Therefore, the parametric function between critical tube diameter and the critical energy would be E_c=16^-1πρ₀v_CJ²Id_c³.