The serrated yielding phenomenon is studied in an Al-Mg alloy during the tensile tests at ambient temperature and constant strain rate. The temperature field on the surface of the specimen changes with the increase of strain. An infrared camera is used to observe and record the temperature variations at very high time and space resolutions. According to these thermal data, the formation and propagation of the deformation bands (type A, B) are discussed and some characteristic parameters of the bands such as orientation, bandwidth, and apparent velocity are also obtained. Furthermore, the plastic deformation in the band is quantified from heat transfer equation. Both experimental and computational results show that elastic shrinkage deformation exists outside the band only during the formation of the type B band. Based on this, whether or not shrinkage deformation exists outside the band is proposed as a new standard to discriminate type A and B bands.