  The existence of Bloch flat bands provides an facile pathway to realize
strongly correlated phenomena in materials. Using density-functional theory and
tight-binding approach, we show that the flat bands can form in twisted bilayer
of hexagonal boron nitride ($h$BN). However, unlike the twisted graphene
bilayer where a magic angle is needed to form the flat band, for the polar
$h$BN, the flat bands can appear as long as the twisted angle is less than
certain critical values. Our simulations reveal that the valence band maximum
(conduction band minimum) states are predominantly resided in the regions of
the moir\'{e} supperlattice where the anion N (cation B) atoms in both layers
are on top of each other. The preferential localization of these valence and
conduction states originate from the chemical potential difference between N
and B and is enhanced by the stacking effects of N and B in both layers,
respectively, as demonstrated by an analysis of the energy level order of the
$h$BN bilayers with different stacking patterns. When these states are
spatially localized because regions with a specific stacking pattern are
isolated for moir\'{e} supperlattices at sufficient small twist angle,
completely flat bands will form. This mechanism is applicable to other twisted
bilayers of two-dimensional polar crystals.
