Objective To investigate root canal curvatures of human maxillary first premolars in buccolingual directions. Methods One hundred extracted permanent maxillary first premolars were used. After access preparation, a K- file was inserted into the canals and gently advanced until it reached the apical foramen. Then standardized periapical radiovisiography（RVG）images were taken from mesiodistal direction. The angle of curvature was determined according to the method described by Schneider. The radiu of curvature and length of the curved part of the root canal were calculated. Results 1）The total 50.00%（88/176） of root canal curves were in buccolingual directions, and 53 curves were showed in apical 1/3 of root region. 11.93%（21/176） of S- shaped canals were detected, and the first curves were mostly showed in middle 1/3 of root region, the secondary curves were totally showed in apical 1/3 of root region. 2）There was no difference of curvatures between buccal and lingual root canals, but the secondary curves of S- shaped canals were more complicated than other root canals. 3）There was no correlation between the angle of curvature and the radius of curvature（r=0.002, P>0.05）, but significantly correlation between the angle of curvature and length of the curved part of the root canal（r=0.489, P
