A method for fabricating flexible display device includes the following steps. Firstly, a rigid substrate is provided. Secondly, a sacrificing layer is formed on the rigid substrate. Thirdly, an element layer is formed on the sacrificing layer. Fourthly, the sacrificing layer is etched by a gas and then gasified, so that the element layer is separated from the rigid substrate. Then, the element layer is adhered to a flexible substrate. Because products generated by the sacrificing layer reacting with the gas are gases, the products can be removed by air exhaust for simplifying process. Thus, the cost of the process of fabricating flexible display device can be decreased.