Abstract: In this talk, I will briefly highlight our work on understanding the 3D-morphology of magnetic fields in star-forming molecular clouds. We first proposed and demonstrated a new method to determine the line-of-sight strength and direction of magnetic fields in star-forming regions using Faraday rotation measurements. We applied this method to four relatively nearby filamentary molecular clouds and for two of them, we found clear evidence that the line-of-sight magnetic field reverses across these clouds. I will briefly introduce the method and explain the techniques we used to infer the 3D-magnetic-field morphologies in these clouds (using our line-of-sight results).