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.