A tensile test is proposed to evaluate the thermal fatigue resistance of solder alloys. The test is based on the strain rate change method to obtain the strain rate sensitivity index, m. The m value is obtained at various strains during the tensile test. The plots of m and strain where m is measured showed a linear relation; therefore, the m value at zero strain, m(0), and the gradient, k, are obtained by extrapolation. The m value at zero strain related to the coarseness or fineness of solder alloy microstructure; m(0) becomes lower with coarsening of the microstructure. The solder alloys with low m(0) and low k have excellent thermal fatigue resistance when compared with alloys with high m(0) and high k. m(0) and k would be good measures to estimate the thermal fatigue properties of solder alloys; highly resistant alloys have low m(0) and low k.