Transmission electron microscopy (TEM) investigation have been carried out on the microstructure of InGaAs buffer layers grown on GaAs (001) substrates using four different strategies via metalorganic vapor phase epitaxy (MOVPE). As compared with the quality of InGaAs layer grown on directly GaAs substrate, the growth on linearly-graded InGaAs (LG-InGaAs), step-graded InGaAs (SG-InGaAs) and stain-layer superlattice InGaAs/GaAs (SLS-InGaAs/GaAs) yielded good structural quality buffer layers. The number of dislocations (misfit, threading and mixed dislocations) investigated by cross-sectional TEM was found to be reduced in the InGaAs buffer layers. The generation of dislocations was found to be dominated in the graded regions. This means that the LG-InGaAs and SG-InGaAs layers were relaxed due to the large lattice-mismatch between InGaAs and GaAs, resulting in generation of a large number of dislocations. On the other hand, for the InGaAs buffer layer on SLS-InGaAs/GaAs, a high density of dislocations was observed in the superlattice regions. In fact, density of dislocations was decreased in the InGaAs buffer layer grown on the InGaAs/GaAs superlattice. This demonstrates that the strained-layer superlattice exhibits some filtering of threading dislocations. Also, the strain-relaxation will be discussed in comparison between the InGaAs buffer layers on GaAs, LG-InGaAs, SG-InGaAs and SLS-InGaAs/GaAs. We found that the grading technique has the advantage of spreading MDs with depth throughout the InGaAs layer and must achieve full relaxation. On the other hand, the SLS technique can be some filtering of TDs. Thus, we suggest that a combination of the LG and SLS techniques is promising method to achieve high-quality strain-relaxed InGaAs buffer layers for the large lattice-mismatched system. We also show that the use of the InGaAs pseudo lattice-matched substrate is an effective method to fabricate a thick lattice-matched InGaAsN layers with higher optical and structural qualities necessary for the development of the optoelectronic devices such as semiconductor lasers and multijunction (MJ) solar cells.