In wireless communications systems, harmonics and unwanted frequency other than designed one becomes a dominant factor in limiting quality and capacity of transmitted or received signal. Rectangular spiral antenna (RSA) is designed in order to obtain the required parameter responses at 2.45 GHz for Wifi application. RSA of length 16cm and width 20cm is designed on the substrate dielectric material FR-4 having dielectric constant =4.3 and thickness 1.6 mm. Conventional rectangular spiral antenna without DGS has very low bandwidth. The defect ground structure (DGS) has gained much attention over the last few years for its ability to enhance the bandwidth and gain. The antenna without DGS has multiple frequencies which are 1.3734, 2.0886 and 2.6538 other than the desired frequency. The DGS also effectively suppress harmonics frequencies of the spiral antenna, shows a better result compared to the conventional antenna.