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In this study, a set of testing methodology was developed for the microtensile specimens. With the methodology, the effect of testing conditions on the mechanical properties of an underfill material was investigated at various strain rates from 10−/s to 10−/s over a wide temperature range from −40 to 240°C. It was found that the testing conditions, in particular, the testing temperature affect the measured properties greatly. The relationship between the testing temperature and the mechanical properties exhibits a nonlinear characteristic and shows three trends. More seriously, the mechanical properties were found to have a sharp change around the glass transition temperature (Tg). In contrast, the mechanical properties were found to have an approximate relationship with the logarithmic strain rate. More over, the fracture surfaces of the tested specimens were analyzed by scanning electron microscope (SEM). The fracture features were employed to explain the experimental observations.