In this communication, the design and working principle of a wideband microstrip to substrate integrated coaxial line (SICL) planar transition for millimeter-wave applications is presented. In the proposed work, along with tapering of microstrip line, two triangular slots in the top ground plane are utilised to compensate for the discontinuity arising at the microstrip-SICL junction minimize reflections over a wide bandwidth. To confirm the theoretical claims, the proposed microstrip to SICL transition is fabricated using standard multilayer PCB process. The measured results of the prototype indicate return loss better than 14 dB with a maximum insertion loss of 1.78 dB for DC to 40 GHz band (19 dB bandwidth up to 20 GHz with insertion loss less than 0.71 dB). The modeling of proposed transition is presented versatility of the design is illustrated for different dielectric constant thickness. Effortless integration of SICL with the widely used microstrip transmission line simplifies the feeding process of SICL based broadband antennas microwave circuits. © 2013 IEEE.