We constructed a plant phloem-specific expression iaaM vector to study the effect of IAA over-expression on phloem development. The vector was constructed by recombining the tryptophan monooxygenase gene (iaaM) with the phloem-specific promoter and the sucrose synthesis gene promoter of Arabidopsis thaliana. The recombinant was transformed into Nicotiana tabacum and several transgenic plants were screened out. Most of the transgenic plants showed abnormal morphologies as leave curling and delayed development and the seedlings grew slower compared to the wild control. The section observation of the stem showed that more secondary xylem and phloem cells were produced in transgenic plants compared to the wild-type. In addition, there were many adventitious roots abnormally grown out in the transgenic stems and numerous root hairs and lateral roots occurred in transgenic root.