  We theoretically study the piezoelectric effects in moir\`e graphene systems.
Since the strain couples to the electrons in the system as a pseudo vector
potential, which has opposite signs for the $K$ and $K'$ valleys of graphene,
its effects on the two valleys with opposite Chern numbers do not cancel out,
but adds up. As a result, some components of the piezoelectric tensor in these
systems, which typically have non-trivial topology in their flat bands, are
nearly quantized in terms of the valley Chern numbers. Such a conclusion is
verified by numerical calculations of the in-plane piezoelectric response of
hBN-aligned twisted bilayer graphene, twisted bilayer-monolayer graphene, and
twisted double bilayer graphene systems using both continuum model and
atomistic tight-binding model. We find that by tuning the vertical displacement
field and/or twist angle, which may induce gap closures between the flat bands
and remote bands in these systems, plateau shapes of the piezoelectric response
are obtained, with abrupt jumps across the topological phase transitions. We
propose that such nearly quantized piezoelectric response may serve as a direct
experimental probe for the valley Chern numbers of the flat bands in moir\'e
graphene systems.
