Abstract: The high inhibition effect of 2,2,6,6-tetra-methyl-4-hydroxyl piperi-dine-oxyi (TMHPO.) on radical polymerization of acrylic acid, initiated by benzoylperoxide, has been studied. The results showed that the inhibition effect was predominant when the amounts of TMHPO. were approximately equal to 0.1-0.3% of those of acrylic acid, and became more prominent especially when TMHPO. was the incorporated with 1,4-benzo-quinone and phenothiazine. For presumably, these were a synergistic action between TMHPO. and other inhibitors such as 1,4-benzoquinone and phe-nothazine.