  Within a quantum molecular dynamics model we calculate the largest Lyapunov
exponent (LLE), density fluctuation and mass distribution of fragments for a
series of nuclear systems at different initial temperatures. It is found that
the $LLE$ peaks at the temperature ("critical temperature") where the density
fluctuation reaches a maximal value and the mass distribution of fragments is
best fitted by the Fisher's power law from which the critical exponents for
mass and charge distribution are obtained. The time-dependent behavior of the
LLE and density fluctuation is studied. We find that the time scale of the
density fluctuation is much longer than the inverse LLE, which indicates that
the chaotic motion can be well developed during the process of fragment
formation. The finite-size effect on "critical temperature" for nuclear systems
ranging from Calcium to superheavy nuclei is also studied.
