Objective:To evaluate dental and skeletal changes following slow maxillary expansion with quad helix using Cone-Beam computer tomography (CBCT).Methods:13 patients(5males and 8 females,mean age 14.38 ±2.22 years)requiring maxillary ex-pansion as a part of their comprehensive orthodontic treatment were included.Each patient had CBCT images taken pre-(T1 )and post-(T2)maxillary expansion with quad helix.Changes of the distances between bilateral canines,first premolars,second premo-lars,first molars,the width of basal bone and palatal suture were measured.paired t-test results were analyzed using SPSS 17.0 soft-ware.Results:The distances between the 4 bilateral teeth increased by (2.47 ±1.05)mm,(2.97 ±1.90)mm,(2.79 ±1.21) mm and (3.15 ±1.15)mm,the apical distances decreased by (1.19 ±0.40)mm,(2.12 ±0.68)mm,(2.02 ±0.65)mm and (1.34 ±0.63)mm,respectively.The inclination of the first molars were decreased by (4.45 ±2.86)°and (4.02 ±1.45)°on the left and right side respectively.The width of basal bone and palatal suture increased by (2.37 ±0.96)mm and (1.21 ±0.50)mm respectively,the differences between T1 and T2 were all statistically different(P <0.001).Conclusion:Quad helix expands maxil-lary arch by greater dental changes than by skeletal changes.