A comparative study between via-holed and via-free back-to-backGCPW-MS-GCPW (Grounded Coplanar Waveguide-Microstrip lines) transitions is reportedin this paper. According to simulation results, both via-holed and via-freetransitions on commercial benzocyclobutene polymer 20-µm film show a bandwidth over57 GHz. Bandwidth of optimized via-holed transitions increases with the via-holediameter, up to 75 GHz with 300 µm via-hole diameter. The via-hole freetransition achieves experimentally an ultra-broadband from 2 GHz to 78 GHz with an insertion loss of only0.5 dB thanks to the copper metallization thickness of 2 µm.In addition, these measurement results are in perfect agreement with thesimulation results. These via-free and via-holes transitions are very usefuland requested in component packaging, on-wafer measurements of microstrip basedmicrowave integrated circuits, and also in the interconnections in hybridcircuits including both microstrip and coplanar structures.