Ephemeral gullies (EGs) are important erosion features that could account for as much as 70% of total erosion on the Loess Plateau of China. However, few quantitative studies have been conducted in the field on the impact and mechanism of upslope sediment runoff in EG erosion processes. A series of field rainfall simulation experiments were conducted on an artificial EG to investigate the impacts of upslope inflow with two flow rates (15 and 30 L min(-1)) and five sediment concentrations (20, 40, 80, 120, and 160 g L-1) on runoff and erosion processes within the EG. The results showed that the lower flow rate of 15 L min(-1) increased soil loss by 26% compared with the higher inflow rate (30 L min(-1)). The lower inflow rate triggered greater sheet or rill erosion, whereas the higher rate mainly strengthened the incision of the main EG channel and led to increasing gully erosion. The channel shoulders expanded laterally very little, especially under the higher flow rate. Inflow sediment concentration had a major influence on EG erosion processes. With increasing sediment concentration, EG erosion and total slope erosion first increased, and then decreased. When sediment concentration exceeded 120 g L-1, the inflow sediment began to be deposited, and 120 g L-1 can be regarded as a critical value of EG erosion. These results are beneficial to establish a soil erosion model and to control ephemeral gully erosion in loess areas.