A doublet flow is the circular jet added the annular suction flow mounted on the same axis by using a double coaxial pipe. This flow was proposed for the purpose of diffusion control of a circular free jet. The doublet flow was measured by using PIV. The effects of the diameter ratio of suction pipe to injection pipe on the time mean characteristics and the unsteady characteristics of two dimensional flow field were examined. From the analysis of the time mean characteristics, it is clarified that the fluctuating velocity increases by adding the suction flow in all cases of the diameter ratio. Therefore, it is found that the doublet flow has the superior effect of the diffusion compared with a circular free jet. And the cause of the increase of the fluctuating velocity is clarified by the analysis of the unsteady characteristics. Furthermore. the experimental equations which can predict the fluctuating velocity were proposed.