The purpose of this study is to investigate the effect of premolar extractions on tooth size discrepancy. A total of 213 plaster casts were divided into a Bolton big (BB), a Bolton normal (BN) and a Bolton small (BS) group according to the Bolton normal value plus or minus one standard deviation. The Bolton overall ratio of each group was measured and compared before and after hypothetical premolar extractions. The Bolton overall ratios after premolar extractions were smaller than those before extraction in each group. Some of the patients in the BN group and BB group were moved into the BS group and BN group after premolar extractions. A tooth size discrepancy occurred in some patients with normal overall ratios after premolar extractions, but a tooth size discrepancy might be corrected in some patients with big overall ratios after premolar extraction.