Abstract: To explore the influences of arbuscular mycorrhizal fungi (AMF) and P level on plant root system architecture, tomato seedlings were inoculated with AMF strain Rhizophagus irregularis BGC JX04B under two P levels, and the influences of AMF and P level on lateral root (LR) formation of tomato seedlings were studied. Results indicated that the promoting effect of AMF on plant biomass was not evident, but significantly decreased the root to shoot ratio of plants. AMF significantly increased the primary root length but decreased the 1^st order LR length and interacted with the mycorrhizal colonization period. AMF significantly lowered the 2^nd-3^rd order LR number and the ratio of 2^nd order LR number to 1^st order LR number, but did not significantly affect the 1^st-2^nd order LR density. High P level (50 mg·kg^-1 P) significantly promoted the plant growth and decreased the root to shoot ratio of plants. It had no significant effect on the primary root length and the 1st order root length, but significantly enhanced the 1^st-3^rd order LR number and the ratio of 2^nd order LR number to 1^st order LR number, increased the 1^st-2^nd order LR density. It suggested that AMF and P level did not share a common mechanism to influence the LR formation of tomato plants. The influence of high P level may depend on its promoting effects on nutrient uptake and plant growth, while the influence of AMF is more complex. Furthermore, the interaction between AMF and mycorrhizal colonization period implies the possible involvement of carbohydrate distribution (sugar signaling) in the regulation of root system architecture by AMF.