  We show that for the thermal spectrum of Hawking radiation black hole's
information loss paradox may still be present, even if including the
entanglement information stored in the entangled Minkowski vacuum. And to avoid
this inconsistency, the spectrum of Hawking radiation must be nonthermal. After
reconsidering the derivation of Hawking effect, we find that the thermal
spectrum is actually resulted from the geometric optics approximation in
deriving the Bogolubov coefficients. When treated a little more accurately, we
obtain some nonthermal spectrum for the Hawing radiation, which reduces to the
thermal one in the geometric optics approximation.
