The structures of Ta/Ni81Fe19 and Ni8l Fel9/Ta are commonly used in magnetoresistance multilayers. It is found that the thickness of dead layer in Ta/Ni81Fe19/Ta was about 1.6±0.2 nm. X-ray photoelectron spectroscopy (XPS) was used to study the interfaces of Ta/Ni81Fe19 and Ni81Fe19/Ta. The results show that there is a reaction at the two interfaces: 2Ta+Ni=NiTa2, which caused the thinning of the effective NiFe layer. Furthermore, this reaction could also explain the phenomenon that the dead layer thickness of spin valves multilayers prepared by MBE is thinner than those prepared by magnetron sputtering.