Aqueous free-radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in the presence of potassium persulfate (KPS) as thermal initiator was studied by differential scanning calorimetry (DSC). Effect of temperature, monomer, and initiator concentration on polymerization rate was studied. Obtained recorded data represented order of reaction with respect to monomer greater than unity (1.56). Among the theories presented order of reaction more than one, the gage effect theory in the range of monomer concentration (0.966-2.410 mol/L) had a good agreement with mechanism of AMPS polymerization. The order with respect to KPS was 0.52 which was consistent with classical kinetic theory. Calculated amount of activation energy in overall rate of reaction was 83 kJ/mol K.