A multimode bandpass filter working at X-band is designed and developed, and both the simulated and measured results are presented. A constant-impedance stub loaded with three stepped- impedance stubs constitutes the main structure of this filter. Surface current distribution maps are utilized to acquire parameters which influence the distribution of transmission poles. By regulating the parameters of stepped impedance stubs, the performance of this filter can be optimized. For further enhancing the suppression of stop band, a pair of parallel coupled lines are joined up in series at both sides to introduce a transmission zero at 14 GHz. The measured results show that the designed filter has the center frequency of 9.76 GHz, the bandwidth of 2.4 GHz, the rectangular coefficient at 30 dB/3 dB of 1.63, the relative bandwidth of 25%, and most of the insertion loss and the return loss of the passband are respectively less than 1 dB and higher than 15 dB, which agree well with the simulated results.