Abstract: Co-rotating vortices are generated with the deployment of flaps when aircrafts are taking off or landing. The flap vortices and the tip vortices interact with each other and also influence themselves as they roll up. Then they merge together to form a pair of trailing wakes as they flow down stream. The vortex wake is very dangerous for the following aircraft. Wake vortices also influence and limit the capacity of airports. Hence the mechanism of co-rotating vortices mergence is of great importance. It can be used to predict even control of merging location and vortex dissipation. The merging mechanism of co-rotating vortex pair is numerically simulated using large eddy simulation, the nonlinear merging process at different separating distance and different angles of attack are analyzed, and the results are in good agreements with the experimental results.