We review recent progresses on in-situ observation of lattice relaxation of III-V lattice-mismatched system and analyses of defect properties in III-V-N solar cell materials. We found that there were five phases during the InGaAs growth on GaAs substrate. The transition point of the dominant dislocation behavior could be determined precisely. We also found that compositionally step-graded InGaAs/GaAs(001) buffers with overshooting (OS) layers were effective to control the strain of the top layer from tensile to compression. To understand the defect properties that dominate the electrical property of CBE-grown GaAsN films, we characterized deep levels in CBE-grown GaAsN films by DLTS. In this characterization, a well-known electron trap E1 (Ec-0.33eV) center in n-GaAsN and p-GaAsN was confirmed to be non-radiative recombination center by using double-carrier pulse DLTS.