With a large number of multi‐input multi‐output (MIMO) antennas embedded in the smartphone, much higher data throughput can be obtained for the smartphone in the MIMO operation. Furthermore, in order to be operational in heterogeneous networks, such as the mobile network and the wireless wide area network (WLAN), dual‐band operation of the MIMO antennas is desirable. However, owing to very limited space available in the smartphone, it has been a great challenge to dispose more MIMO antennas therein, not to mention the dual‐band MIMO antennas. To address this challenge, we present a compact decoupled 3.5/5.8 GHz (3400–3600/5725–5875 MHz) dual‐band building block formed by dual inverted‐F/loop antennas and its application for building eight dual‐band MIMO antennas for the mobile and WLAN operations. Details of the dual‐band building block and the fabricated eight MIMO antennas are described. The channel capacity of the eight MIMO antennas in an 8 × 8 MIMO system is also calculated and discussed. The obtained MIMO channel capacities are respectively about 37 and 42 bps/Hz in the 3.5 and 5.8 GHz bands, much higher than the present fourth‐generation (4G) 2 × 2 MIMO capacity (about 10 bps/Hz or less). The proposed eight MIMO antennas will hence be promising for future smartphone applications, such as in the fifth‐generation (5G) communications.