In the present paper, coherent structures of the supersonic mixing layer are investigated by direct numerical simulations of a spatialy developing free shear layer, We examined numerical results of the velocity components, u, v, w simulated by Fouillet and Lesieur : Reynolds number Re=δ_ω0ΔU/ν=1500, Mach number =1(M₁=1 and M_c=0.3), (U₁+U₂)/2=0.7, U₁-U₂=0.6, and the hyperbolic-tangent velocity profile is perturbed by weak three-dimensional white noise. We find that the coherent structures in the supersonic mixing layer at M_c=0.3 are three dimensional and have rib structures with strong vorticities of ω_y and ω_x similar to those in the incompressible mixing layer. The helical pairing structures found in the rib structure generate a coherent 'upwash' and 'downwash' fluid motion in the supersonic mixing layer. The regions corresponding to high rates of dissipation are found in the rib structures.