Angle-dispersive x-ray diffraction experiments using synchrotron radiation and an imaging plate were carried out for ferroelectric KH2PO4 (KDP) and KD2PO4 (DKDP) under high pressures and at room temperature. We found that in DKDP phase IV transforms to phases VI and VI' at about 7.5 and 19 GPa, respectively, in the compression process and that during decompression phase VI' transforms to phase VII at about 5 GPa before transforming back to phase II at 2 GPa. The crystal systems of phases VI and VII were analyzed to be orthorhombic and monoclinic, respectively. In case of KDP, we observed that phase VI' becomes amorphous by compression up to 73 GPa at room temperature. During decompression, the amorphous phase recrystallizes to phase II, demonstrating that the pressure-induced amorphization is reversible. The volume changes associated with the phase transition of IV-VI for DKDP and KDP at the transition pressure are found to be 13.7% and 14.3%, respectively.