We discuss energy diffusion and acceleration of charged particles by large amplitude magnetohydrodynamic (MHD) waves by test particle simulations, paying particular attention to the effects of phase correlation of the waves. In quasi-linear theory, the wave phases are assumed to be random, but in reality, interaction between different wave modes may produce phase correlation. When the wave phases are strongly correlated, the waveform appears as spatially localized traveling wave packet. These wave packets can efficiently accelerate charged particles, as they are repeatedly mirror reflected by oppositely propagating wave packets: a process analogous to the Fermi acceleration. ¿ 2000 American Geophysical Union