A procedure for determining the ceiling temperature in radical polymerization was proposed. By means of this procedure, the ceiling temperature in radical polymerization of 2, 6-dimethyl-phenyl methacrylate was determined. It was noted that the introduction of the methyl groups in the 2- and 6-positions of phenyl methacrylate lowered remarkably the ceiling temperature as the results of steric hindrance. This polymer was consistently found to undergo easily thermal decomposition into the monomer. These results were explained by increased steric requirement between α-, 2-, and 6-methyl groups in the polymer.