Both the morphology and architecture of fine roots vary according to the stage of development，which directly influences their ability to absorb water and nutrients from the soil，and consequently，influences plant growth． To understand the effect of N fertilization on the fine root architecture and growth dynamics of Bothriochloa ischaemun，six N treatments (0，0． 02，0． 04，0． 08，0． 16，0． 32 g/kg) were carried out under greenhouse conditions． Using a scanning method we monitored root extension and radial growth dynamics． The scanned pictures were analyzed to obtain root length，diameter，and surface area． Dynamic cluster analysis was then used to sort the root orders according to the morphological factors． After scanning，we flushed the roots，and then dissected the individual roots according to the branching orders． The fine root samples were scanned to determine root length and diameter，and specific root length． The root was oven-dried at 65 ℃ for 48 h before being weighed． The results showed: (1) the mean fine root diameter and mean root length tended to increase (p ＜ 0． 05) with ascending root order，while both specific root length (SRL) and total root length decreased (P ＜ 0． 05); (2) the mean root diameter and mean root length increased with increasing N fertilization，however，the SRL，total root length and root biomass decreased with increasing N，except the diameter of the 1st order root; (3) N fertilization increased the root extension rate (RER) and increased radial growth rate (RGR)． The RER and RGR for all four root orders first increased to peak levels，which then decreased，and finally held constant．