Polarization effects in self-organized In0.25Ga0.75N/GaN quantum dots grown by RF-plasma-assisted molecular beam epitaxy have been studied experimentally. The quantum dots show fairly small dot size and high dot density. The excitation power-dependent photoluminescence characteristics of these dots have been carefully investigated. Strong photoluminescence (PL) was observed between 472 and 500 nm, depending on the excitation power density. Blueshift of the PL peak position indicates the presence of built-in electric fields in the quantum dot heterostructures. Analysis of the measured data leads to a built-in electrical field of 1.7 MV/cm without excitation, which is in good agreement with theoretical predictions. (C) 2008 Published by Elsevier B.V.