Self-decoupling antennas are designed in this paper. The decoupling principle is based on a weak field created by a T-shaped feeding line (TSFL) and the antenna element itself. When arranging another identical antenna in the weak field, high isolation between two antennas can be achieved without adding extra decoupling structure. First, the weak field of a single antenna is described and analyzed by the equivalent circuit. Then, the adjacent antenna is placed in the weak field to obtain a high isolation level of 51 dB between two-element MIMO antennas. Furthermore, a four-antenna linear MIMO was designed, which also achieved high isolation. With the merits of simple structure and efficient decoupling, these self-decoupling antennas are promising for MIMO UHF RFID Systems.