Shifts of functional traits are important because phenotypic responses of species to environmental changes caused by natural and anthropogenic disturbances are fundamental in determining the risk of population extinction. This study tested the effect of forest thinning on the body shape and male genital size of an endemic ground beetle species Carabus masuzoi (Imura and Satô 1989) (Coleoptera, Carabidae) in cypress plantations started approximately 30 yr ago in central Taiwan. The beetles were sampled and compared from 1) natural broadleaf forest, 2) non-thinned cypress plantation, and 3) 45% thinned cypress plantation. Female prothorax length from the non-thinned plantation was significantly greater than that of the natural forest, and the 45% thinned plantation had a higher frequency of the small (S-type) and a lower frequency of the large (L-type) male genitalia than in the natural forest. The results indicated that, within a short ecological time frame, the prothorax shapes and male genital sizes of C. masuzoi populations might respond to changes induced by different forest types and forest thinning, respectively. We hypothesized that the difference in prothoracic shape was related to locomotory ability in forest understories, whereas the changes in male genital sizes might have been a result of different levels of intraspecific sexual selection, random effects of population fluctuations/dispersal, or pleiotropy.