Molecular dynamics simulation was carried out to investigate structural phase transitions of ice in high-pressure phases. Transition from ice VIII to ice VII and further to ice X was obtained on applying pressure. Transition from ice VIII to ice VII on heating was also successfully reproduced. Ice VII thus obtained was orientationally disordered, while ice VII in high pressure was highly ionized, which is in good agreement with experiments. Another series of MD simulations were done to investigate high-pressure and high-temperature forms of ice. First, on heating, transition from amorphous ice to ice X was obtained, which is an order-disorder transition with respect to the oxygens. Then, on further heating, ice X was transformed into another form of ice in which the oxygen lattice forms an fee structure and protons are almost in a gas state. Although the KKY potential that we have used in the present MD simulation is the one originally developed for low-pressure ice and water, it turns out that it can be applicable to the high-pressure phases of ice with predictions in agreement with experiments.