  The observed hemispherical power asymmetry in cosmic microwave background
radiation can be explained by long wavelength mode (long-mode) modulation. In
this work we study the prospect of the detection of this effect in the angular
power spectrum of 21-cm brightness temperature. For this task, we study the
effect of the neutral Hydrogen distribution on the angular power spectrum. This
is done by formulating the bias parameter of ionized fraction to the underlying
matter distribution. We also discuss the possibility that the long mode
modulation is companied with a primordial non-Gaussianity of local type. In
this case, we obtain the angular power spectrum with two effects of primordial
non-Gaussianity and long mode modulation. Finally, we show that the primordial
non-Gaussianity enhances the long mode modulated power of 21-cm signal via the
non-Gaussian scale-dependent bias up to four orders of magnitude. {Accordingly,
the observation of 21-cm signal with upcoming surveys such as the Square
Kilometer Array (SKA) is probably capable of detecting hemispherical power
asymmetry in the context of the long mode modulation.
