Symmetry is a key to unlock exotic phenomena in condensed matter physics. Particularly in magnetic systems, the symmetry analysis enables deep understanding of magnetic ordering and electron transports. In this talk, I will give three examples showing how symmetry analysis can help understand and design magnetic materials of interest. First, I will show the prediction of giant perpendicular magnetic anisotropy (PMA) in iron monolayer on top of III-V nitride substrates. The magnitude of PMA is up to 50 meV, in contrast to 1meV in conventional Fe-based thin films.