Recently, in some silicon carbide single crystals, some micropipes associated with screw dislocation have been observed by X-ray topography and the strain field around them produced images similar to those of screw dislocations with a very large Burgers vector, about 667 nm. The radius of the hole in the centre of the micropipe is less than 10 'm. This value and the theoretical predictions by Frank (about 7.8 mm) using the Burgers vector magnitude show a large discrepancy. In this paper we present Atomic Force Microscopy experiments around this kind of defects. The Burgers vector magnitude of the screw dislocation and the value of the radius have been measured by this technique. Not only one dislocation, but several have been observed around the micropipe. We concluded that it is in better agreement with the Frank theory modified by Cabrera and Levine concerning kinetic effects during the growth.