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Long filamentary structures are commonly found inside giant molecular clouds. Some extend to more than 10-pc long and some have clumpy structures along the clouds. High resolution imaging of filamentary clouds shows that long filamentary clouds also compose of smaller filamentary substructures. Chains of dense cores are found to be forming along some of these substructures. The clumpy and chain-of-cores appearances of some filamentary molecular clouds indicates that these clouds probably are gravitational unstable and are undergoing the fragmentation process at the early stage of star formation. From polarization mapping of magnetic fields around filamentary molecular clouds, observers also find that magnetic fields are generally roughly perpendicular to the long axes of the clouds. Could such field structure help in supporting filamentary cloud from gravitational collapse? In this talk, I present the magnetic field structures of the filamentary molecular clouds in the OMC-3 and OMC-4 regions in Orion A cloud complex and the L1495/B211 region in Taurus cloud complex from our recent polarization mapping using HAWC+ on SOFIA. I shall discuss the physical properties of the filamentary clouds in these regions from our stability analysis based on the observed magnetic field structure, column density, and velocity dispersion. We shall see what the role of magnetic fields is in the stability of filamentary clouds in these regions and, with the help of numerical simulation, how viewing angle may affect our interpretation of the conditions in molecular clouds.