Because wire fixation continues to be used extensively in the practice of orthopaedic surgery, despite a high incidence of wire breakage, understanding the mechanism of this failure is of important clinical interest. The aim of this study was to investigate the failure of cerclage stainless steel wire using an in vitro cyclic loading device. A stainless steel testing fixture consisting of two half cylinders with a combined diameter of 2.5 cm was mounted in a servo hydraulic testing machine. Specimens of number 18 gauge (0.97 mm diameter) and number 16 gauge (1.22 mm diameter) 316L stainless steel wire were mounted around the two half cylinders in a cerclage manner using three different fastening methods: a uniform symmetrical twist, a knot twist, and a square knot. Single-load-to-failure and cyclic load tests were performed under controlled tensile displacement. The cerclage wire system fastened with a twist resulted in failure at loads significantly lower than systems fastened with the knot twist and the square knot. Cyclic loading of the wire fastened with twists also showed decreased fatigue properties when compared to those fastened with the knot twist and the square knot. In all tests, the 16-gauge wire was found to be clearly superior to the 18-gauge wire. For both wires, fatigue strengths at 100,000 cycles were only 30-37% of the static ultimate strength. These results show that wire diameter and fastening system are two important factors affecting the mechanical properties of the resulting fixation.