The micro and ultrastructure of the roots of Robinia pseudoacacia were studied by optical and electron microscope. The results showed when Robinia pseudoacacia were cultured in the medium without iron, the inoculated seedlings’root epidermal or exodermal cells developed many transfer cells with wall in growths and these transfer cells lasted until the development of Robinia pseudoacacia’s root nodule. While the ones without inoculation only developed few transfer cells. And, when aseptic Robinia pseudoacacia were cultured in the medium with iron there was no wall ingrowth in root epidermal cells. So rhizobia was the main induced actors of transfer cells, and the lacking of iron was only the secondary factors during the development of root nodule.