Optical and morphological properties of self-assembled InAs quantum dots (QDs) covered by In_xGa_1-xAs strain reducing layers (SRL) with different thicknesses (2, 4, 6 and 8nm) and compositions (x=0.13, 0.18 and 0.30) were investigated. Photoluminescence from InAs QDs shows the dependence on indium mole fraction and thickness of the overgrown In_xGa_1-xAs SRL.Improvement in PL intensity and narrowing of PL width up to 26meV occurred together with a red shift of up to 138nm when the QDs were coved with 6nm of In_0.18Ga_0.82As. Also, we found that when the total amount of InAs deposited to form the QDs and the SRL was larger than a critical value of around 6MLs, the surface roughness increased and the PL intensity decreased drastically.