Abstract: This paper is concerned with the relation between the catalytic activity and pore structure of five kinds of clay from different places used as catalyst for the polymerization of cottonseed oil acid. The amount of N₂ adsorption on the clays was measured by means of gas chromatography at the temperature of liquid nitrogen. The types of the pore structure of clay were estimated from the isotherms of the adsorption and desorption as well as from the DTA and SEM bbservations. The calculated results show that all the samples are rich in the porosiy of medium size. since it gives the highest yield of dimeric octadecadienoic acid, the Jilin clay which has the pore diameter of d= 30-40Å showed a better specificity in the formation of the dimeric acid.