The microstructure and sequence distribution of 3

4-polyisoprene obtained with Fe(acac)3-phen-AlEt3 catalyst have been examined by C-13 NMR spectroscopy. The results show that 3

4- cis-1

4 and trans-1

4 contents calculated respectively from aliphatic and olefinic carbons are in agreement on the value and it was found that monomer units of 3

4-polyisoprene exist in a small amount of "head-head" and "tail-tail" structure in addition to "head-tail" linkages.