Abstract: Various changes of locust roots were observed by optics microscope after inoculating with rhizobia.There was no deformation all the time in the root hair of locust which didn’t inoculated with rhizobia.The deformed root segments inoculated with rhizobia after 3～4 days were dyed with methylene blue.It was observed clearly that the position where rhizobia invaded the seedling of locust at the top of root hair and then entered the cortex.