Forming limit curve (FLC) is an important tool for assessing formability of steel metal. It is commonly obtained from experiment, theoretical calculation and finite element method (FEM) simulation. In this study, the FLC of a seamed tube hydroforming is established by combining the failure criterion of strain increment ratio and FEM simulation. The numerical method is verified by tube bulge tests. Then the sensitivity studies are carried out to evaluate the effect of the geometrical features of seamed tube on its formability by numerical approach. Results show that the changes of the formability with the geometrical features of a seamed tube.