In this paper, using gallium arsenide (GaAs) technology, a revolutionary millimeter-wave on-chip ultra-wideband (UWB) bandpass filter (BPF) with greatly enhanced operation performance is demonstrated for the first time. By utilizing quarter wavelength short-circuited lines with an open-circuited interdigital coupled-line and a short-circuited combine coupled-line, it is altered from the traditional wideband BPF structure to create a flexible coupled line based bi-path topology. In comparison to the traditional structure, the suggested topology can efficiently increase passband selectivity with more flexible transmission zeros (TZs), as well as two more transmission poles (TPs) for a flatter UWB passband and a larger return loss (RL) bandwidth. For verification, an on-chip UWB BPF prototype using 0.25μm GaAs pHEMT technology is designed, fabricated, and measured. The measurements show that it has a center frequency of 45.3 GHz with a fractional bandwidth (FBW) of 135%. A good agreement is found between the simulated and measured data.