  Taking the two-dimensional $\phi^4$ theory as an example, we numerically
solve the deterministic equations of motion with random initial states.
Short-time behavior of the solutions is systematically investigated. Assuming
that the solutions generate a microcanonical ensemble of the system, we
demonstrate that the second order phase transition point can be determined
already from the short-time dynamic behavior. Initial increase of the
magnetization and critical slowing down are observed. The dynamic critical
exponent z, the new exponent $\theta$ and the static exponents $\beta$ and
$\nu$ are estimated. Interestingly, the deterministic dynamics with random
initial states is in a same dynamic universality class of Monte Carlo dynamics.
