The terpolymer of disodium itaconate (DIA), acrylamide (AM) and sodium 2-acrylamido-2-methyl-1-propane sulfonate (SAMPS) was synthesized through free-radical polymerization, and characterized using FTIR and TGA methods. The IR spectra of DIA-AM-SAMPS terpolymer confirmed that there was no olefinic band at 1635-1620 cm(-1), while the TGA results revealed that the terpolymer was of higher thermal stability than the SAMPS homopolymer. The filtrate volume reduced with increase of the terpolymer concentration before or after the aging test. The rheology properties of both fresh-water mud and salt-water mud were improved by DIA-AM-SAMPS terpolymer, and apparent viscosity (eta(alpha)); plastic viscosity (eta(b)) and yield point (tau(0)) of salt-water mud reached the smallest values at 1.2%, of the, terpolymer concentration after the aging test. The particle size data demonstrated that only a small change of the clay particle size occurred before and after the aging test at 220degreesC. This further confirmed the thermal stability of the terpolymer-clay dispersion from another point of view.