The present work was focused on the effect of structure of coaxial nozzle (especially for the thickness of central tube) on the instability and breakup of liquid jet. Air-blast atomization of the liquid jet was carried out in several coaxial atomizers. Based on photographs of the liquid jet recorded by high speed digital camera, wave frequency of liquid surface in the near-field was discussed. The results showed that the frequency increases with the velocities of the gas jet and the liquid jet, and the thickness of central tube had an important effect on the liquid instability. A mathematical model that can quantify the effect of central tube thickness on the wave frequency was obtained by analyzing the influence of central tube on the evolution of vorticity layer thickness in the nozzle near-field region. This model would provide significant theory foundation to the design of nozzles. (C) 2013 Elsevier B.V. All rights reserved.