The concept of Defected Ground Structures (DGS) has been developed to improve the characteristics of many microwave devices. For this purpose the DGS is also used in the microstrip antenna for some advantages such as antenna size reduction, mutual coupling reduction in antenna arrays etc… In this paper the defected ground structure (DGS) has been employed to miniaturize a microstrip patch antenna and to shift the resonance frequency from an initial value of 10 GHz to a final value at 3.5 GHz, without any change in the dimensions of the original microstrip patch antenna. This antenna is designed on a FR-4 substrate with dielectric constant 4.4 and thickness 1.6 mm and its size is 27 × 30 mm 2 . The antenna is designed, optimized, and miniaturized by using CST MW.