The in-plane dynamic behaviour of hexagonal aluminium honeycombs has been studied in this paper by means of finite element simulation using ABAQUS. The influences of honeycomb cell wall thickness and impact velocity on the mode of localised deformation and the plateau stress are investigated. Deformation modes in the X1 direction change significantly with different values of cell wall thickness and impact velocity. Three deformation modes are observed. They are summarised in a mode classification map and two empirical formulae are obtained for critical velocities at which deformation mode changes. Two deformation modes are observed in the X2 direction. The plateau stresses are related to the cell wall thickness by a power law for a given velocity and they are proportional to the square of velocity for a high velocity. An empirical formula for plateau stress at high impact velocities is derived in terms of the cell wall thickness and velocity.