  The sensitivity to the sterile neutrino mixing at very short baseline reactor
neutrino experiments is investigated. If the reactor core is relatively large
as in the case of commercial reactors, then the sensitivity is lost for $\Delta
m^2 \gtrsim$ 1 eV$^2$ due to smearing of the reactor core size. If the reactor
core is small as in the case of the experimental fast neutron reactor Joyo, the
ILL research reactor or the Osiris reactor, on the other hand, then sensitivity
to $\sin^22\theta_{14}$ can be as good as 0.03 for $\Delta m^2 \sim$ several
eV$^2$ because of its small size.
