A novel CPW-fed millimeter wave slot antenna for broadband circular polarization and unidirectional radiation with multilayer 3D structures is discussed. The antenna is fabricated based on micro-electro-mechanical systems micro-fabrication technology. Through a special short-circuiting stub perturbation method, the circular ring slot antenna generates circularly polarized radiation and achieves a broad impedance bandwidth of 20% with voltage standing-wave ratio (VSWR) ⩽ 2 and circularly polarized bandwidth of 17.6% with AR ⩽ 3 dB. By adding a special wave guiding slot structure and wave cutoff slot structure, a novel approximate perfect magnetic conductor surface, the antenna radiates unidirectionally and the peak gain increases from about 5.2 dB to 7.5 dB.