An approach is proposed to characterize bi-axial fatigue resistance of polymer plates. The method proposed here can detect a change of mechanical properties of polymers (especially ductility) due to fatigue loading, before any visible crack is generated. A decrease in ductility by fatigue loading has been reported for polymers subjected to uni-axial fatigue stresses. Whether a similar phenomenon occurs under bi-axial fatigue stresses is not known at present. In this study, a new bi-axial testing device was designed and built that is capable of applying equal bi-axial forces to cruciform specimens in a cyclic mode. This paper details the test method, including design and instrumentation of the bi-axial testing device, specimen design, and the procedure to determine the load for a desired bi-axial stress state. Poly(acrylonitrile-butadiene-styrene) (ABS) was used as the sample material. Preliminary results reported here show that the ABS's ductility can be reduced significantly by applying 500 cycles of bi-axial fatigue stress at a level that is about 50% of its tensile strength. The results also indicate that such a level of bi-axial fatigue stress has induced extensive rubber particle cavitation, though it is yet to clarify whether the particle cavitation has led to the ductility drop. The study concludes that the methodology proposed in this paper can be used to evaluate bi-axial fatigue resistance of polymer plates for the purpose of materials evaluation. (C) 2005 Springer Science + Business Media, Inc.