Countercurrent shear can be used in engineering applications to control flow structure and mixing. In the present paper a planar countercurrent shear flow is studied numerically using Large Eddy Simulation. Mean flow characteristics at a primary-jet Reynolds number of 14700 are studied for three values of a key parameter, the secondary to primary jet mass flow ratio (m s /m p), chosen to match reported measurements. The predicted flow results of time averaged flow field, Strouhal number and turbulence characteristics are compared with the experimentally available data. A change in instability modes is observed at mass flow ratios above m s /m p = 0.27 with more than 100% increase in the turbulence levels and distinct changes in spectral characteristics. Detailed spectral records along with landau modeling are used as evidence of existence of self excited global mode beyond a critical velocity ratio i n the flow.