Transitional characteristics of a plane jet excited by wake of a two-dimensional cylinder installed in nozzle contraction has been investigated by FFT and phase-averaging techniques. The slot width h of the nozzle was fixed to 15mm. The diameter of the cylinder d was 2mm, and the location of the cylinder inside the nozzle contraction Xc was 90 mm from the nozzle exit. The exit velocity was 10.1m/s, and Reynolds number based on the slot width was 1.0×104. In this setup condition of the cylinder, two distinctive spectral peaks (f1 and f2) centered at 1/2 the excitation frequency were observed. And also inharmonic phenomenon indicated by presence of beat frequency of the two frequencies was observed. The results show that the jet under inharmonic excitation exhibits flapping motion at frequency f1-f2, and the jet starts to rolling-up to form vortex street at x/h>8.