Abstract: In the present paper,an experiment of the mantle uplifting and lateral extension is conducted under three different boundary conditions using a ductile/brittle double-layer physical model. The experiment results show that when the model is fixed on two sides,uplifting could cause stretch and faults with very small lateral flow. When the model are half or completely open,uplifting could cause very large extension,mainly in the direction of the faults early produced. Evidently the faults early formed in uplifting would create a favourable condition for the subsequent extension and the uplifting would accelerate the flow of ductile layer towards the direction of open boundary to promote further extension. In addition,some passive updoming of lower ductile layer is found where faults occurin the upper brittle layer in our models. It is therefore clear that no large extension should arise from mantle uplifting without favourable boundary conditions.