A pure dimer were prepared by GPC from the products of selective dimerization of α-pinene, then with HPLC the number and the relative content of dimeric isomers in the dimers were measured. And the effect of catalysts and polymerization conditions on the number and the relative content of dimeric isomer in the dimers was investigated. Generally, four peaks of dimeric isomer were observed, but the relative ratio of these peaks changed with the catalysts and reaction conditions. Besides the regular dimers with molecular weight (MW) 272, several dimeric isomers with 270 and 274 of α-pinene were observed for the first time by GC-MS combine analysis. On the basis of the data of the 1 H-NMR, 1 3 C-NMR, UV and the fragment ions of MS, main molecular structures of these dimeric isomers were proposed.