This paper describes the results of experimental studies concerning the near-field vortex structure and dynamics of a coaxial jet. The effect of excitation and circular jets on the vortical structure in the inner and outer shear-layer was studied. The flow visualization and the measurements of mean and fluctuating velocities by a hot-film probe and the particle image velocimetry (PIV) technique, were carried out in an open water tank. For the excitation of the initial vortex frequency, it is found that the vortex-pairing event is promoted by the forced by the forced excitation and it results in the rapid expansion of the jet width and the increase of velocity fluctuation and the entrainment rate. It is certified that the vortex-pairing event in the shear-layer is affected by the excitation of the relevant jet. For the velocity ratio near unity, the dominant peak frequency becomes the initial vortex frequency independent of the forced excitation frequency.