  Twisted bilayer graphene (TBG) exhibits fascinating correlation-driven
phenomena like the superconductivity and Mott insulating state, with flat bands
and a chiral lattice structure. We find by quantum transport calculations that
the chirality leads to a giant unidirectional magnetoresistance (UMR) in TBG,
where the unidirectionality refers to the resistance change under the reversal
of the direction of the current or magnetic field. We point out that flat bands
significantly enhance this effect. The UMR increases quickly upon reducing the
twist angle and reaches about 20\% for an angle of 1.5$^\circ$ in a 10 T
in-plane magnetic field. We propose the band structure topology (asymmetry),
which leads to a direction-sensitive mean free path, as a useful way to
anticipate the UMR effect. The UMR provides a probe for chirality and band
flatness in the twisted bilayers.
