A special selective electrochemical etching based equipment is presented which is appropriate for in-situ observation of the defect structure. The working of the set-up is demonstrated on the InGaAs/GaAs (001) heteroepitaxial systems where the epitaxial layer thickness was above the critical layer thickness. By incremental layer removal, the depth profile on the dislocation density was mapped. The measured defects density is inversely proportional to the layer thickness and corresponds to theoretical model.