The structural stability of zinc blende (ZB) structured GaN thin films on ZB-GaN(001) and ZB-SiC(001) substrates is systematically investigated based on an empirical potential, which incorporates electrostatic energy due to covalent bond charges and ionic charges. The calculated energy difference between pure ZB-GaN and mixed structure of wurtzite (W) and ZB-GaN (ZB-W-GaN) on ZB-GaN(001) implies that ZB-GaN is stabilized up to the film thickness of 554 ML. Furthermore, similar calculations incorporating misfit dislocations (MDs) on ZB-SiC(001) show that ZB-GaN pseudomorphically grown on ZB-SiC(001) initially changes its structure to ZB-GaN with MD at 28 ML in good accordance with critical thickness of misfit dislocation generation of 32 ML estimated by People-Bean formula. The ZB-GaN with MD is successively transformed into ZB-GaN with MDs at 294 ML and finally ZB-W-GaN at 865 ML. These results suggest that MD generation crucially affects the stability of ZB-GaN. [DOI: 10.1380/ejssnt.2005.507]