This paper presents a MIMO antenna using two elements in the shape of a ring with a rectangular stub for RFID applications. The proposed MIMO antenna is multiband with center frequencies at 2.45GHz, 3.5GHz, and 5.4GHz. Mutual coupling between the two antenna elements is improved by placing a U-type resonator. The isolation between the antenna elements is more than -15 dB on these three frequency bands. The diversity gain is 10dB, while the ECC is less than 0.05. The antenna is fabricated on a low-cost substrate, FR4, and its dimensions are $31 \times 31 \times 0.8$ mm³.