A systematic investigation of the effects of oxygen contamination in Al on microstructures and stress‐migration failures in Al lines has been made using Al lines intentionally contaminated with oxygen to various degrees. The results of stress migration tests at 150 °C, 200 °C, 250 °C and 295 °C show that stress‐migration characteristics, i.e., void shape and statistical lifetime distribution, are strongly influenced by oxygen‐contamination levels. Two characteristic failure types were derived from the results: one, the low‐oxygen type, is characterized by open failures caused by large (∼2 &mgr;m2) wedge‐shaped voids, small (∼0.15) logarithmic standard deviations, and a large (∼1.2 eV) activation energy, and the other, the high‐oxygen type, is characterized by open failures caused mostly by crack‐like voids and large (≳1) logarithmic standard deviations. Instantaneous failure rates at operating temperatures, estimated from the storage test results, seem to be negligible until the wearout period begins for the low‐oxygen type, while those for the high‐oxygen type seem to be unacceptably high in the early and random failure periods. Therefore, reducing oxygen contamination is useful for lowering the stress‐migration failure rate at operating temperatures.