The ultrafine coal water slurry (CWS) with the particle size of 1-10 mu m, ash content of 1-2%, solid concentration of 50% is a promising substitute fuel for diesel oil. The effects of pore fractal structures of three ultrafine CWSs on their rheological behaviors and combustion dynamics were studied in this paper. When the pore fractal dimensions of Yanzhou, Huainan and Shenhua ultrafine CWSs increase, their apparent viscosities all increase and the increase extents gradually enlarge with decreasing shear rates, while their ignition temperatures and apparent activation energies all decrease. For example, when the pore fractal dimension of Yanzhou coal increases from 2.31 to 2.43, the CWS apparent viscosity at a low shear rate of 12 s(-1) increases from 75 mPa s to 2400 mPa s, and that at a high shear rate of 100 s(-1) increases from 80 mPa s to 820 mPa s. Meanwhile, the ignition temperature of Yanzhou CWS decreases from 445 degrees C to 417 degrees C at a heating rate of 12.5 degrees C/min, and the apparent activation energy decreases from 104 kJ/mol to 32 kJ/mol. (c) 2008 Elsevier Ltd. All rights reserved.