Failure of organic packages is often accelerated by mechanical failure of an underfill designed to protect the solder joints. The crack growth rate of an underfill under thermal cycling is crucial for package reliability and lifetime prediction. Mechanically-induced fatigue crack growth results have been used in package reliability studies, but thermally-induced fatigue crack growth data for underfills does not exist. Thermal fatigue experiments are complex to perform, since it is thermal loading that grows underfill cracks in actual packages. We describe the fatigue crack growth behaviors of an underfill measured by applying thermal fatigue stresses to obtain low-cycle fatigue da/dN vs. thermal stress intensity factor plots. The number of cycles to failure or fast fracture was found to depend strongly on thermal loading. We compared thermal fatigue data with thermal fracture toughness results, and found that the final crack length after thermal fatigue crack growth compared well with the critical crack length of thermal fracture data.