In this paper, the Koch fractal shape is applied for the first time to microstrip bandpass filters integrated on a high-resistivity Si substrate. By using this method, the second harmonics of the filter can be suppressed to about -40 dB. Conventional microstrip coupled line filters are popular in RF front ends, because they can be easily fabricated and integrated with other RF components. However, they typically have large second harmonics that can cause unwanted interference. Without any additional filters, the proposed Koch shape filters have suppressed the 2nd harmonics by about -40 dB, so they can be used in systems with stringent harmonic suppression requirements.