We have grown In-rich InGaN epilayers on InN by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), and the role of thin InGaN interlayers inserted between InN and InGaN were investigated in structural and optical properties of In-rich InGaN. InGaN interlayers (InGaN-ILs) were grown at the same growth condition of upper InGaN except for growth temperature. InGaN layer directly grown on InN was not flat owing to suppressed adatom mobility at N-rich V/III ratio, even though a part of surface was well developed to flat (0 0 0 1) plane. Insertion of 1 nm InGaN-IL did not give any change to the surface morphology of InGaN. On the other hand, 3, 10, and 30 nm InGaN-ILs apparently changed InGaN's morphologies to be more continuous, and the luminescence property was also largely improved. However, 10 and 30 nm InGaN-ILs slightly deteriorated the crystal quality of InGaN probably caused by thick interlayer containing a lot of defects. Therefore, we found that 3 nm LT-InGaN was very effective to improve the morphology of InGaN not deteriorating the crystal quality of InGaN. And it was also noticeable that the strain of InGaN was reversed from tensile to compressive by InGaN-IL, which is one of the positive approaches to suppress surface cracks. (c) 2006 Elsevier B.V. All rights reserved.