  The thermal instability of the giant graviton is investigated within the BMN
matrix model. We calculate the one-loop thermal correction of the quantum
fluctuation around the trivial vacuum and giant graviton respectively. From the
exact formula of the free energy we see that at low temperature the giant
graviton is unstable and will dissolve into vacuum fluctuation. However, at
sufficient high temperature the trivial vacuum fluctuation will condense to
form the giant graviton configuration. The transition temperature of the giant
graviton is determined in our calculation.
