Purpose : One of the required information in rehabilitating the occlusal plane in dentistry is the radius of Monson’s sphere , which like other anatomical indices varies in different races. The purpose of this study was to determine the radius of the Monson’s  sphere in a group of  Iranian. Materials & Methods  : 45 Iranian subjects (19 males & 26 females ) aged from 18 to 25 were selected for this study . The x,y,z coordinates of cusp tips of all  mandibular teeth except for the third molars obtained  with a 3-dimensional digitizer , were used to  derive a spherical model  of the occlusal surfaces’ curvature . From the best interpolating sphere , the radii of the Monson’s sphere was computed . Results : Although all of the computed variables were greater in men , the occlusal curvature of mandibular arch was not influenced by gender . The mean radius of the Monson’s  sphere was 111.5 mm ( 121 mm in men and 104 mm in women which was closer to the classic value of 4 inch , confirming Monson’s observations ) . Conclusion  : The average radius of the Monson’s sphere in this study was greater than the classic 4 inch value proposed by Monson and this result may suggest racial differences in the normal dental arch form that is important in occlusal rehabilitation. Although more researches are necessary in future to improve treatment results.