DYNA - a hydrodynamic simulator for two-valley semiconductor devices. An efficient semiconductor device simulation package DYNA, is introduced. A new relaxation time evaluation scheme for two-valley semiconductors is used to account for the electron dynamics under nonuniform fields and for the dependence of the electron mobility on the impurity scattering. Some robust solution methods are used in the simulator for treating the system of highly nonlinear equations. The simulation results for a nonuniformly- dropped GaAs MESFET show good agreement with that of the Monte Carlo device simulation.