Use of external fixators in undeveloped countries has been limited due to their high cost. Consequently, one of the aims of our group is the design, construction, and evaluation of low cost external fixators. In this paper we present the mechanical characterization of an Ilizarov fixator composed of rings made of aluminum, which is light, cheap, easy to form, radiolucid and anticorrosive. The main purpose was to determine the 3-D stiffness of the fixator (Duda et al., 1998) in order to be able to evaluate interfragmentary movement. We hypothesize that this fixator has the ability to limit shear and allow axial movements that are beneficial to bone healing (Goodship et al., 1985; Kenwrigh et al., 1991). Given the amount of work involved to experimentally determine the 3-D stiffness matrix of an external fixator we used simple computational models that were calibrated experimentally for a couple of load cases.