A kind of microstrip patch antenna that similar to Koch fractal structure with a square notched structure was fabricated. Through adding square notched structure on both sides of microstrip patch antenna to study the radiation property. Along with the increase of fractal order number and the square notch number, the microstrip patch antenna shows various features. When the number of the square notch adds to a certain amount, the microstrip patch antenna can suppress the high harmonic interference. The reflection coefficient s11, voltage standing wave ratio and radiation pattern of the fractal antenna have been obtained using Hfss13.0 software. The results proved that when the fractal order number is 1, 2, the antenna have a role of frequency multiplication; while the fractal order number is 10 20, the antenna can suppress the interference of higher harmonic. Finally, a 10 order fractal antenna has been fabricated and measured. The experimental results and simulation results have a good agreement with each other.