The densification kinetics and grain growth behavior of an undoped tungsten powder during spark plasma sintering (SPS) were investigated under the pressure of 40 MPa and constant heating rate of 100 degrees C min(-1). Two stages of the sintering process were clearly identified: densification without grain growth at the low temperatures (1200-1450 degrees C) and grain growth without much further densification at higher temperatures (1500-2000 degrees C). A creep model was applied to determine the densification mechanisms involved in the densification stage, which can be elucidated by evaluating the stress exponent (n) and the apparent activation energy (Q(d)) from the densification rate law. It shows that a boundary diffusion governs the densification process at low effective compaction stresses (n = 1.5, Q(d) = 14057 +/- 12 kJ mol(-1)), while grain boundary diffusion and dislocation motion both operate at higher effective compaction stresses (n = 3, Q(d) = 302.48 +/- 24 kJ mol(-1)), which is confirmed by transmission electron microscopy observation. During the final-stage of sintering, the fast grain growth mechanism was suggested as surface diffusion. (C) 2017 Elsevier B.V. All rights reserved.