Investigation of hydrostatic strain and change in dislocation densities of edge and screw dislocations have been studied before and after passivation of amorphous silicon nitride (a-Si3N4) layer on Al0.3Ga0.7N/GaN heterostructure by non-destructive high resolution x-ray diffraction (HRXRD) analysis. Group III-nitrides materials are quite attractive for high power, high frequency, high temperature applications at microwave frequency due to their high band gap and high break down voltage. Epitaxially grown III-nitrides films give rise to high strain and as well as defect densities due to various parameters such as lattice mismatch, dopant, unintentional impurities, thermal expansion coefficient etc. The high dislocation density arises due to unusually large lattice mismatch in hetero epitaxial films and also between the epitaxial layer and substrate. Al0.3Ga0.7N/GaN group III-nitride heterostructure has been grown by plasma assisted molecular beam epitaxy (PAMBE) technique on c-plane sapphire substrate and an amorphous silicon nitride(a- Si3N4) film of 40 nm thickness as a passivation layer has been deposited on the AlGaN layer by plasma enhanced chemical vapor deposition (PECVD) technique. The highly strain sensitive nature of HRXRD gives a remarkable strain-stress and structural analysis of heteroepitaxial films. The well-known Bragg’s law and Vegard’s law have been used to evaluate hydrostatic strain of Al0.3Ga0.7N/GaN heterostructure which is expressed in terms in-plane strain (εin) and out of plane strain (εout). The out-of-plan and in-plane strains before and after passivation are evaluated from HRXRD (ω-2θ) diffraction profile across (002) plane and asymmetric (ω-2θ) diffraction profile across (105) plane respectively. After passivation strain is increased at heteroepitaxially grown Al0.3Ga0.7N/GaN structure, which imply the reduction of surface states at the interface of dielectrics and AlGaN layer. The edge and screw dislocation densities was found out from full width at half maxima (FWHM) of triple axis Omega_Rel (002) and skew symmetric (102) diffraction profile respectively. After passivation on Al0.3Ga0.7N/GaN heterostructure dislocation density is found to increase due to the formation of dislocation boundary at the interface of passivation layer and next epitaxial layer. This boundary generates dislocation microstructure such as dislocation pile ups. After passivation due to reduction of surface states issues and enhancement of hydrostatic strain, the Al0.3Ga0.7N/GaN heterostructure would be more effective to increase strain induced polarization charge effects as well as 2DEG carrier density for high density and high power HEMT applications.