A user terminal device which is bendable, comprising: a flexible display configured to be divided into a first area and a second area in response to the user terminal device being bent, a bending part configured to link two parts of the user terminal device, thereby enabling the user terminal device to be bent with reference to an axis, wherein each of the first area and the second area are bendable around the axis of the bending part of the user terminal device; a bending sensor configured to detect a bending state of the user terminal device; and a controller configured to, in response to the flexible display, which is in an inactivation state in which the flexible display is turned off, being bent about the axis to a step-bending state, control the flexible display to activate the first area to display a screen while the second area is maintained in the inactivation state, control the flexible display to display an execution screen of an application on the activated first area while the user terminal device is maintained in the step-bending state based on the detected bending state, and, in response to the user terminal device changing from the step-bending state to an unbent state, control the flexible display not to display the execution screen of the application on the first area by deactivating the activated first area; and control the flexible display to display a screen including a UI element used in the application in the activated first area in response to the bending state changing from the step-bending state to a pre-bending state; wherein in the step-bending state the user terminal device is maintained in a bent state without external pressure; and wherein in the pre-bending state the user terminal device automatically returns to a previous bending state without external pressure; wherein the UI element comprises at least one of an image, a text, or a moving image for receiving a user input and providing feedback according to the received user input. a flexible display configured to be divided into a first area and a second area in response to the user terminal device being bent, a bending part configured to link two parts of the user terminal device, thereby enabling the user terminal device to be bent with reference to an axis, wherein each of the first area and the second area are bendable around the axis of the bending part of the user terminal device; a bending sensor configured to detect a bending state of the user terminal device; and a controller configured to, in response to the flexible display, which is in an inactivation state in which the flexible display is turned off, being bent about the axis to a step-bending state, control the flexible display to activate the first area to display a screen while the second area is maintained in the inactivation state, control the flexible display to display an execution screen of an application on the activated first area while the user terminal device is maintained in the step-bending state based on the detected bending state, and, in response to the user terminal device changing from the step-bending state to an unbent state, control the flexible display not to display the execution screen of the application on the first area by deactivating the activated first area; and control the flexible display to display a screen including a UI element used in the application in the activated first area in response to the bending state changing from the step-bending state to a pre-bending state; wherein in the step-bending state the user terminal device is maintained in a bent state without external pressure; and wherein in the pre-bending state the user terminal device automatically returns to a previous bending state without external pressure; wherein the UI element comprises at least one of an image, a text, or a moving image for receiving a user input and providing feedback according to the received user input.