This paper presents a third harmonics up pressed bandpass filter on GaN MMIC (Monolithic Microwave Integrated Circuit). By analysing the voltage distribution at fundamental frequency and third harmonic of the resonators, suitable coupling region between the feeding lines and resonators can be selected. In this way, we make the coupling coefficient at third harmonic to be zero while the coupling coefficient at fundamental frequency is not zero. Therefore, the third harmonic can be suppressed by the discriminating coupling scheme while the fundamental mode responses are unaffected. Meanwhile, to improve the selectivity, source-load coupling is employed to introduce transmission zeros near the passband. For demonstration, a millimeter-wave filter with wide sropband is implemented. The filter is designed based on the technology of GaN MMIC with center frequency of 33.5 GHz and insertion loss of 3.2 dB. The circuit area of the filter is only 410 µm × 210 µm, featuring very compact size.