Purpose: Monofilament, ployester, and multifilament are the common types of tennis string. In the current study, we intended to discuss the mechanical properties with different string diameters, 15 gauge (1.35mm), 16 gauge (1.30mm), 17 gauge (1.25mm), and 18 gauge (1.20mm), in those strings. Method: The experiments were performed in a laboratory with controlled temperature and humidity. We used the universal testing machine to obtain the tensile load-displacement curve for all strings. The stiffness at 22.7kg, 25kg, 27.3kg, and 29.6kg in the load-displacement curve were calculated, and the nonparametric test was used to compare the differences between different string diameters. Results: The monofilament showed the highest ductility, and the polyester with diameter of 1.30mm showed less ductility. For the same type of strings, different diameters would preserve similar characteristics of the load-displacement curve, expect for the polyester. In general, as the diameter of the string increased, the stiffness would also increase. The statistically significant differences in stiffness were found among different diameters of the same type of the strings. One exception was found for the multifilament, the string with 1.20mm diameter showed similar stiffness with the diameter of 1.35mm. Conclusion: The results of this study can be a reference for the novice or expert players in choosing suitable string. And it also can provide the information about properties with different string diameter for optimal exercise performance.