Abstract: A series of ultimate loading tests on two types of uplift piles with an enlarged base with effective pile lengths of 19 m and 30 m are carried out. For each type, three test piles are loaded to failure. The deformations of the test piles at the top and toe are continuously monitored throughout the tests. For the 19 m-long piles, the axial force of pile shaft is measured as well. Load-displacement curves, distribution of axial force of pile shaft and shaft resistance are investigated. The monitoring data and numerical analysis results show that the uplift capacity of uplift piles with an enlarged base is composed of shaft resistance of the uniform section and uplift resistance of the under-reamed part. When a pile is pulled up, the shaft resistance of the uniform section works firstly, subsequently followed by the uplift resistance of the under-reamed part. At the ultimate state, the uplift resistance provided by the under-reamed part accounts for about 50% and 35% of the total uplift capacity for the 19 m-long and 35 m-long piles. It is also found that the influence of the under-reamed base on the development of shaft resistance is not significant. The uplift resistance of the under-reamed base mainly comes from the vertical component of the normal force of the soil around the base, which takes a proportion of approximately 70% at the ultimate state.