This paper provides preliminary results on the closed-loop control of a spatially developing mixing-layer induced by two parallel incident streams with different velocities. The goal of the control law was to stabilize the flow around a desired state (known to reduce mixing). The way to achieve this flow control consists of linearizing the Navier-Stokes equations about the desired state, spatially discretizing the resulting linear system and determining the feedback gain according to an optimal control law. Actuations were located in the input boundary of the system. State of the flow was assumed to be reconstructed from image sensors. The control law has been validated on a realistic non-linear system (Navier-Stokes solver). More precisely, these simulation results have shown that perturbations can be efficiently rejected.