An authentication method for user equipment (UE) and LIPA network entities is applicable to a cross-LIPA communication environment having an UE end, a visiting LIPA network entity end (LIPAV), and a home LIPA network entity end (LIPAH). The UE end successfully registers to a core network (CN) via the LIPAV, thereby attaining mutual trust relationship between the UE end and the LIPAV. The UE end successfully registers to the CN via the LIPAH, thereby attaining mutual trust relationship between the UE end and the LIPAH. The LIPAH requests the UE end via the LIPAV for successfully re-authenticating the CN, thereby attaining mutual trust relationship between the LIPAV and the LIPAH.