From the perspective of Shannon theory and its extensions, this talk is devoted to evaluating the technique potentials of future 6G mobile communication system. Firstly, the classic Shannon theory framework, including performance tradeoff between block-length, data rate and reliability, is briefly summarized, and the limitations of its application in the contemporary mobile communication system are addressed. Secondly, the multiple-input-multiple-output (MIMO) extension of the classic Shannon theory is described, which has been playing the fundamental roles in the development of contemporary mobile communication systems. Furthermore, aiming at higher spectrum efficiency and power efficiency, higher reliability and lower latency, and higher frequency band, which are essential indicators of future 6G, the technique potentials are discussed theoretically from the perspective of Shannon theory framework. It reveals that by introducing more antennas together with the innovation of cell free network architecture, and by making effective balance between block length, error probability, data rate, and minimum number of antennas, future 6G technology still has great potential to be improved, but it needs both to make a compromise between system performance and deployment cost, and to carefully make use of the special features of MIMO channels in higher frequency band. Finally, several fundamental issues related to future 6G development are summarized.