This paper proposes a simple yet accurate method for estimating the antenna correlation coefficient (ACC) of a high‐order multiple‐input multiple‐output (MIMO) antenna. The conventional method employed to obtain the ACC from three‐dimensional radiation patterns is costly and difficult to measure. An alternate method is to use the S‐parameters, which can be easily measured using a network analyzer. However, this method assumes that the antennas are highly efficient, and it is therefore not suitable for lossy MIMO antenna arrays. To overcome this limitation, we define and utilize the non‐coupled radiation efficiency in the S‐parameter‐based ACC formula. The accuracy of the proposed method is verified by the simulation results of a 4‐port highly coupled lossy MIMO array. Further, the proposed method can be applied to N‐port arrays by expanding the calculation matrix.