ABSTRACT
<p><b>OBJECTIVE</b>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.</p><p><b>METHODS</b>30 first maxillary premolars and 30 first mandibular premolars were collected. Walls of 4 mm short of the apex were respectively measured before and after post preparation with peeso drills number 1-3.</p><p><b>RESULTS</b>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<0.05). For single-canal first maxillary premolars, the average residual mesial and distal thickness were less than 1 mm after post preparation by number 2 peeso drill. The distal wall had significant differences (P<0.05).</p><p><b>CONCLUSION</b>In post preparation, for the buccal root of two-canal maxillary premolars, the maximum size of peeso drill was number 3, and for the palatine root and first mandibular premolars, the size was number 2, and for single-canal maxillary first premolars, the size was number 1.</p>