In fracture problems, stress intensity factors obtained theoretically or by experimental analyses have been effectively utilized in analytical evaluation of the effect of cracks. The optical method of caustics is a very effective technique for evaluating this factor. Use of the caustics method to determine the stress intensity factors of two-dimensional cracks has been established theoretically and experimentally. However, relatively few experimental analyses of three-dimensional cracks using this method have so far been conducted. In this paper, an experimental technique for determining the stress intensity factor KIII for a three-dimensional elastic solid has been presented by using the method of reflected caustics in combination with the photoelastic stress freezing method. The experimental model is a cylindrical bar with a radial crack along the direction of the axis under the load of pure torsion. In order to obtain a caustic pattern, a slice cut from the frozen model is annealed. The results of this experimental method coincided favorably with the theoretical analysis results of Tweed and Rooke.