In this letter, a dual-band multiple-input-multiple-output (MIMO) antenna array is proposed for portable wireless devices. The MIMO antenna array consists of two back-to-back monopoles elements with an edge-to-edge distance of 0.096 λ -(λ is the free-space wavelength at 2.4 GHz). A shorting strip and an isolation stub are designed to reduce the mutual coupling between the two elements. The MIMO antenna array has a volume of 15 × 50 × 0.8 mm3. It can cover the WLAN 2.4- and 5.8-GHz bands. The measured isolation values (S21) are lower than - 25 and - 20 dB at the 2.45- and 5.8-GHz bands, respectively. The key design parameters are presented, and the parametric study is performed.