Aspheric elements are widely applied in optical systems and the vertex radius of curvature (VROC) is one of the important fundamental parameters of an asphere. We present a method for measuring the VROC of asphere. We use a portable laser tracker to measure the optical interval of the null testing path and then determine the VROC of the asphere through ray tracing. Based on this method, we carry out an accurate measurement. The accuracy can reach up to 0.056 mm on an asphere with VROC of approximately 2 m and the relative error is 0.003%.