The InyGa1−yAs/GaAs superlattice on InxGa1−x.As (x<y) buffer layer was grown by MOCVD. The well layer is under compressive strain and the barrier layer is under tensile strain. However, both layers do not exceed the calculated critical value based on the M−yAs interface was smoother than the InyGa1−yAs/GaAs interface; the Indium composition gradual region at the GaAs/InyGal1−yAs interface was narrower than that at the InyGa 1−yAs/GaAs interface; in InyGa1−yAs alloy layer, the Indium composition near the GaAs/InyGa1-yAs interface was higher than that near another interface. For the first time, we explained the composition profile in this kind of superlattice based on the indium segregation theory. A new strain relaxation model, in which the 30-degree and 90-degree shockley partial dislocations were taken into account under both tensile and compressive strains, was presented to explain the difference of the smoothness between the GaAs/InyGa1−yAs interface and the InyGa 1−yAs/GaAs interface.