In the present study, the interaction effect by inter-rotor spacing of coaxial rotor blades in hovering and forward flight is studied by using a Reynolds-averaged Navier-Stokes CFD flow solver based on mixed meshes. The meshes were composed of unstructured near-body region and Cartesian off-body region. The flux-difference splitting scheme of Roe was used for computing inviscid fluxes on the near-body region, whereas the 7th order WENO scheme was used for the computation of the inviscid fluxes on the off-body region to capture vortex with high resolution. The predicted results of the baseline inter-rotor spacing were compared with the reference results of other researchers for validation. The computed results of three different inter-rotor spacings were compared in terms of the thrust and torque coefficients for each flight condition. The cause of the different interaction effect by inter-rotor spacing between hovering and forward flight was also investigated.