Effects of blockage ratios on the initial instability in the outer mixing layer of annular jet are studied by using hot-wire anemometer. Initial instability frequencies are measured at x/Da=0.75, y/Do=0.5 with diameter ratios Di/DO=0.33, 0.43, 0.5, 0.67 where Do, Di is the outer jet diameter, center-body diameter respectively and Da is (Do-Di)/2. It shows that the initial instability frequencies are proportional to 3/2th power of exit velocity, Ue, irrespective of blockage ratios. Velocity power spectra are measured in the outer mixing layer along the downstream at the jet exit velocity, Ue = 8 m/s. It clearly shows that the evolution of fundamental and sub-harmonic components. Maximum turbulence intensity is measured also. It grows linearly upto x/Do=0.25 in the log scale irrespective of blockage ratios, however, the larger blockage ratio shows smaller amplification rate. In the further downstream the linearity ends up and the trubulence intensities saturate where the sub-harmonic components reach maximum, that is, where the vortex pairing occurs.