In this study, ISM-band microstrip square patch antenna with 2450 MHz operating frequency was designed, the S11 reflection coefficient of the antenna and the maximum antenna gain at 9 and 0 angles was determined, and the values of HPBW were calculated. The simulations were performed in Ansoft HFSS and the accuracy of the gain value enhancement in the results was confirmed by CST Studio Suit. Then, by placing the co-planar structure around this patch, the values for different co-planar thicknesses were obtained again. The maximum antenna gain of the co-planar structure in 9 and 0 angles and its effect on HPBW values were compared. According to this, the maximum gain value of 113 x 118 mm microstrip square patch antenna with no co-planar structure was obtained as 0,308 dB and HPBW value was obtained as 55,05 degrees. When these values were obtained again by adding co-planar structure without changing the antenna dimensions, it was observed that the maximum gain value was 0,748 dB and HPBW value was 49,46 degrees. As can be understood from these values, the addition of co-planar structure was observed to have a positive effect on the antenna parameters by causing the increase in Maximum Gain. HPBW values decreased contrast to the gain. Here, it was observed that adding co-planar structure increased the directionality and max gain of the antenna on the radiation pattern, and the 0 angle, in which the max gain occurred, shifted from 90 to 85. It was also observed that a similar effect occurred at different antenna sizes. Traditionally, antenna gain is achieved by increasing antenna dimensions. In this study, it was investigated to increase the antenna gain without increasing the antenna size marginally.