The theoretical part of this work is dedicated to the adaptation of high-resolution transmission electron microscopy for studying III-nitride semiconductors. First, the principle of heterostructure composition evaluation by means of atomic displacement measurement is stated. The comparison of two strain measurement techniques, geometric phase analysis and projection method, is then presented. Finally, the effects of acquisition conditions on the strain measurements were elaborated. The experimental part of this work is dedicated to the characterization of GaN quantum dots (QDs) grown on AlGaN templates. This study revealed several phenomena original for nitride semiconductors. The surface QD shape depends on the GaN layer thickness, whereas the buried QD shape and volume are influenced by the QD capping. Moreover, a phase separation occurs in the AlGaN barriers. To explain the observed phenomena, various models founded on the principle of total energy minimization have been developed. Several approaches, based on the results of this study and aimed for the improvement of the optoelectronic devices properties, are also proposed.