A new mechanism is established and applied to the upper bound analysis of uplift piles in saturated clay. Upper bound solutions of uplift capacities of closed and open-ended piles are analyzed. The new mechanism is verified by comparing with elastoplastic finite- element, existing lower bound and API solutions. Its reliability is assessed through case studies including one centrifuge test and two field tests. The effects of the pile length-diameter ratio, pile surface roughness, non-uniformity of undrained soil strength and length of soil plug on the uplift bearing capacity are considered. A formulation predicting the net bearing capacity of closed-ended uplift piles in clay is proposed. The results show that an approximately linear relationship can be established between the normalized net bearing capacity coefficient and the pile length-diameter ratio for closed-ended piles. As for open-ended pipe piles, the ratio of the net bearing capacity coefficient of open-ended pipe piles to that of closed-end piles tends to increase with the increase of the pile length-diameter ratio. The increase in the length of soil plug leads to an increase in the net bearing capacity coefficient of open-ended pipe piles. Therefore, the effect of soil plug should not be ignored.