A K-band bandpass filter (BPF) using a parallel-coupled air-suspended microstrip on an anodised aluminium substrate has been demonstrated. The air-suspended structure can be easily fabricated by exploiting the vertical etching property of anodised aluminium oxide. To compare the air-suspended microstrip with conventional microstrips, insertion losses of the coupled lines were calculated using semi-empirical equations. The air-suspended microstrip has the advantage of low insertion loss (IL) over that of a conventional RF substrate when the frequency is higher than 25 GHz, although the physical length of the line is longer. The fabricated filter has a centre frequency of 25.3 GHz, a minimum IL of 1.53 dB, and a fractional band width of 14.2%.