A fan-out wafer level package (FOWLP) with double-sided four redistribution layers (RDLs) and the mega pillars connecting the front and back RDLs has been proposed for millimeter-wave applications. A well-matched package-board transition has been designed in this paper. The simulated insertion loss for the transition is about 0.82 dB at 79 GHz, and the simulated return loss is better than 10 dB from 72 GHz to 86 GHz. More importantly, two different measurement methods based on the port reduction technique and the Thru-Reflect-Line (TRL) calibration technique have been proposed to get the S parameters of the transition. Moreover, the feasibility of the two methods has been verified by simulation.