This paper gives a low-cost and simple anti-jamming technique for gain improvement of global positioning system (GPS) receiver in GPS L1 (1.575 GHz) band. For the GPS receiver, a microstrip patch antenna which has right-handed circular polarization (RHCP) is selected as the basic element. The GPS antenna needs to minimize the gain to the horizontal direction and maximize the gain to the vertical direction, because the received GPS signal is extremely small which can be susceptible from the interference to the horizontal direction. To overcome the unwanted jamming signal, therefore, a conical structure is applied around the patch antenna. As a result, the maximum difference between peak and horizontal gain is about 25 dB, when the height (h) and upper diameter (d) of the conical structure are h= λ0 and d=7/5 λ 0, respectively.