Capacity of Multiple Input Multiple Output (MIMO) system degrades significantly in the presence of interferences, especially in the presence of strong directional interferences in military communication scenario. In this paper, the capacity of a MIMO system with directional interferences is investigated. The focus is on the case where the Channel State Information (CSI) is known to the receiver but unknown to the transmitter. Further, attention is restricted to narrowband MIMO system. The capacity of a MIMO system with directional interferences is derived respectively for deterministic and random fading MIMO channels. And the asymptotic lower limit in high Interference-to-Noise Ratio (INR) is also analyzed. It is indicated that the relationship between the numbers of receive antennas and interferences is critical to the capacity in high INR.