The effect of mutual coupling on the capacity of Multiple-Input-Multiple Output (MIMO) antenna systems is demonstrated by considering a linear array of a four element line fed microstrip patch array. This array is designed at ISM band with
frequency of 2.45 GHz on FR4 Epoxy substrate with relative permittivity (er) of 4.4. The simulation results obtained with this array are compared with the results of a similar array having 2 columns and 4 rows spiral Electromagnetic Band Gap (EBG) structure. The spacing between array elements is variable at the transmitting end and fixed at the receiving end with 0.5l. Array impedance matrices are obtained from simulation results generated by the Ansoft designer software and then used in the calculation of the required coupling matrix. The one-ring channel model is used to compute the correlation between paths and mean capacity of the MIMO system for equal power (EP) and waterfilling (WF) schemes at SNR of 20 dB. A maximum mutual coupling reduction of 17 dB is achieved as a result of using spiral EBG structure. For element spacing in the range (0.48l - 0.6l), which is suitable for the design of antenna arrays, the mutual coupling reduces the correlation level from the ideal case by about 0.15 and the mean capacity by (0.1 – 0.42) b/s/Hz. The reduction of the mutual coupling by spiral EBG structure improves the correlation and the mean capacity that their values are reaching the ideal.