Abstarct: The presence of huge internal electric fields in nitride semiconductors causes for reduction in the efficiency of related electronics and optoelectronics devices. In this thesis we have tried to explain this effect in more details through two parts: experimentally and theoretically. In the experimental part we have studied the existence of stress and strain in a Si-doped GaN (2 )/sapphire structure grown by MOCVD method and tried to model these parameters. The presence of stress in our sample supported by E2 Raman shift mode is about 1.25 cm-1. To find the amount of biaxial basal plane stress we performed two techniques, XRD and film bending by profilometer. XRD technique confirmed that the GaN laxyer has a hexagonal structure with the lattice constants of Å and Å. Our theoretical calculations show this laxyer suffer a biaxial stress ( ) of about 0.175 ± 0.003. The profilometer data analyses also lead to a compatible value of 0.12 ± 0.04 GPa. These parameters altogether suggest the relation cm-1/GPa for this laxyer. In theoretical part firstly we have evaluated compressive strain in GaN/InxGa1-xN interface and related piezoelectric polarization and field by changing indium mole fraction (x). Our calculations indicates that the piezoelectric field in such system increases to 4 MV/cm for x=0.25. A theoretical analysis showed that using this model and including the effect of intrinsic spontaneous field, originating from weak crystal symmetry in wourtzite structures, successfully explains the experimental data reported for the amount of total electric fields in GaN/InxGa1-xN/GaN single quantum well samples. It is well-known that the presence of such internal fields lead to undesirable properties of Quantum Confined Stark Effect (QCSE), one of the main reasons in limiting the efficiency of light emitting devices baxsed on nitride semiconductor nanostructures. In fact this effect causes a reduction in the intensity as well as a red shift in the peak position of luminescence spectrum in such devices. In the second theoretical part we determined the well width (z) dependent effect on red shifted quantum-confined Stark effect (QCSE) in GaN/InxGa1-xN (x=0.13) strained quantum well structures. To explain the corresponding reported data we used a perturbation model to a finite well system in order to find the carriers wave functions as well as their corresponding energy levels. Through this approach we found the first perturbation term (ΔE(1)) vanishes and considering the second term (ΔE(2)) can successfully explain the photoluminescence (PL) peak energy position in such systems. Our analysis also showed that using a variable carrier well-width effective mass dependent, m*(z), for electrons and holes fits much more accurately to the experimental data than a constant effective mass, m*.