Objective The residual dentin thickness of first premolars were measured by anatomical methods before and after post preparation, the data from which can help clinicians to choose proper sizes of peeso drill. Methods 30 first maxillary premolars and 30 first mandibular premolars were collected. Walls of 4mm short of the apex were respectively measured before and after post preparation with peeso drills number 1-3. Results The measurement results of all indexes assumed a normal distribution. For two-canal first maxillary premolars and single-canal first mandibular premolars, the average residual mesial and distal thickness of root walls 4 mm short of the apex were less than 1 mm after post preparation by number 3 peeso drill. The buccal root had nonsignificant differences（P>0.05）. The palatine root and the first mandibular premolars had significant differences（P
