In order to evaluate the sensitivity of defect inspection systems, it is convenient to examine simulated data. This gives the possibility to tune the parameters of the inspection method and to test the performance of the system in critical cases. In this paper, a simple method for the simulation of defects in radioscopic images of aluminum castings is presented. The approach simulates only the flaws and not the whole radioscopic image of the object under test. A 3D flaw is modeled as an ellipsoidal cavity, which is projected and superimposed onto real radioscopic images of an homogeneous object according to the exponential attenuation law for x-rays. The new gray value of a pixel, where the 3D flaw is projected, depends only on four parameters: a) the original gray value; b) the linear absorption coefficient of the examined material; c) the maximal thickness observable in the radioscopic image; and d) the length of the intersection of the 3D flaw with the modeled x-ray beam, that is projected into the pixel. A simulation of an ellipsoidal flaw of any size and orientation can be done in any position of the casting by using the algorithm described in this paper. This allows the evaluation of the performance of defect inspection systems in cases where the detection is known to be difficult.