Pulsating detonations propagating in circular and square tubes were numerically investigated with a two-step reaction model by Korobeinikov et al. The time evolution of the simulation results was utilized to reveal the propagation mechanism of pulsating detonation. Soot track images on the wall and flow features shows cyclic behavior in longitudinal direction including three regions of multi-headed, single-headed, and no detonation. Pulsating detonation shows strong oscillation in the longitudinal direction and periodic behavior as follows. Shock front separates from the reaction front, and the shock wave velocity becomes underdriven. Local explosion occurs near the flame front and develops on inner detonation propagating to the leading shock wave. After penetration of the inner detonation into the leading shock wave, an overdriven multiheaded detonation appears and is gradually transformed into a spinning detonation with diminishing velocity. After a while, the spinning detonation decays, and the shock front finally separates from the reaction front again. Pulsating detonation propagates with repetition of these processes and its velocity varies from underdriven to overdriven in one cycle of pulsation. In order to discuss the one-dimensional features of pulsating detonation, x-t diagrams of cross-sectionally averaged profile are directly compared with x-t diagram of one-dimensional detonation, and the two agree well in terms of periodicity, as observed in the unsteady mechanism of the large-disturbance regime. Characteristic values such as the velocity in the failed regime obtained by the one-dimensional detonation agree with those by the pulsating detonation in circular and square tubes. Therefore, pulsating detonation is essentially a one-dimensional phenomenon and is dominant to longitudinal instability. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.