This survey investigated the relationship between gall morphology and some fitness components in the asexual generation of Cynips quercusfolii L. (Hymenoptera: Cynipidae). Results showed that larger C. quercusfolii galls were formed on the south side of oak trees Quercus infectoria Olivier (Fagales: Fagaceae). Larval chamber diameter in the gall was similar, but gall diameter and gall wall thickness varied with the location of the gall on the tree. Cynips quercusfolii was attacked by parasitoids, and the south-facing galls suffered significantly lower parasitoid attacks. Thickness of gall walls and parasitism rate were negatively correlated. Mean gall diameter and gall wall thickness were significantly larger in south-facing galls than other directions, but the difference in the mean larval chamber diameter was not significant. These results suggest that the position of galls on the tree affected gall wall thickness, and this plays an important role in parasitoid attacks. These results suggest that C. quercusfolii prefer to attack the south side of oak trees, and selection of this side by wasps led to formation of larger galls with thick walls that decreased parasite attack, which will affect growth and survival of wasp larvae.