Abstract: The Zeta potential of thermal reaction samples of Middle East AR were studied by phase analysis light scattering (PALS) Zeta potential analyzer. The samples were confected into the mixture of n-heptane and toluene（1 g/1 g）, and the diluting ratio was (500 mL solvent) / (1 g sample). The results showed that the Zeta potential of ME-AR was –41.8 mV, and the Zeta potentials of 40 min and 100 min reaction samples were 26.9 mV and 31.8 mV. The result showed that the dielectric repulsive strength was not important to the colloidal stability of residue. Moreover, the diameters of colloidal granules of residue were increasing and the polydispersities were decreasing as the thermal reaction going on.