  Two critical end points (CEPs) of the chiral phase transition and the nuclear
liquid-gas phase transition show up at finite baryon chemical potential. The
kurtosis $\kappa\sigma^2$ of baryon number fluctuation on the $T-\mu_B$ plane
is positive on the first-order side and negative on the crossover side along
the phase boundary. The freeze-out line extracted from the heavy ion collisions
crosses between these two phase boundaries, one can observe a peak of
$\kappa\sigma^2$ around the collision energy $5 {\rm GeV}$ near the CEP of the
chiral phase transition, and negative $\kappa\sigma^2$ at low collision
energies due to the CEP of the nuclear liquid-gas phase transition. This
expalains the experimental measurement of $\kappa\sigma^2$ at the collision
energies of 2.4 GeV at HADES and 3 GeV and 7.7-200 GeV at STAR for most central
collision. Thus we propose that the baryon number fluctuation $\kappa\sigma^2$
can be used as a probe of nuclear matter phase structure at high baryon
density.
