A microstrip patch antenna with circular polarisation is presented by using an intrinsically asymmetric 3D-printed substrate. Unlike conventional circularly polarised (CP) antenna, the perturbation for generating CP waves is caused by the asymmetry of substrate, where a pair of triangular air insertions is symmetrically etched in one of diagonals of the square substrate. Besides, a slot is etched on the centre of patch and parallel to its side for good impedance matching. Actually, in consideration of the fact that in 3D printing, the dimensional tolerance for dielectric is less than the one for metal, the presented approach has more superiority than the conventional methods. A prototype is fabricated and measured. Measured results are found in reasonably good agreement with the simulated ones. Stereo lithography apparatus technology is here employed in the 3D printing process.