A plasma simulation code is applied to interpret the instabilities in an expanding planetary nebula. The temperature of the central star of a planetary nebula is assumed as above 50,000 K. Most of the atoms are ionized at this temperature. Since ionization cannot be neglected for such a hot plasma, the electrostatic instability should be taken into account. In the one dimensional electrostatic simulation, Maxwell and Vlasov equations are used and the fast Fourier transform is applied. The calculated drift velocity in the simulation is found comparable with the expansion velocity of a planetary nebula. The linear and non-linear behaviors of the simulated nebular plasma have been investigated in phase space; the simulation results agree with the theory.