A flexible organic light emitting display device comprising: a panel including a plurality of data lines (DL1,..., DLd), a plurality of gate lines (GL1,..., GLg), a plurality of sensing lines (SL1,..., SLk), and a plurality of pixels, wherein each pixel comprises an organic light emitting diode (OLED) and a pixel driving circuit comprising a driving transistor (Tdr) adapted to drive the organic light emitting diode (OLED), the driving transistor (Tdr) having a characteristic that changes when the driving transistor (Tdr) extends or shrinks, each pixel being connected to one of the plurality of data lines (DL1,...DLd), one of the plurality of gate lines (GL1,..., GLg) and one of the plurality of sensing lines (SL1,...SLk); a data driver configured to drive the plurality of data lines (DL1,..., DLd); a gate driver configured to drive the plurality of gate lines (GL1,..., GLg); the data driver configured to supply sensing data voltages during a blank period of one frame period and to output data voltages corresponding to video data during a display period of the one frame; a sensing unit connected to the plurality of sensing lines and configured to determine a current flowing through the driving transistor (Tdr) and determine sensed data (Sdata) accordingly during the blank period and to output sensed data (Sdata) such that pieces of the sensed data (Sdata) vary according to a degree to which the driving transistor (Tdr) extends or shrinks; and a controller configured to determine a characteristic change of the driving transistor (Tdr) based on the sensed data (Sdata) transferred from the sensing unit and to determine whether the panel is bent based on the characteristic change, wherein the characteristic change of the driving transistor (Tdr) consists in a change in mobility or a threshold voltage. a panel including a plurality of data lines (DL1,..., DLd), a plurality of gate lines (GL1,..., GLg), a plurality of sensing lines (SL1,..., SLk), and a plurality of pixels, wherein each pixel comprises an organic light emitting diode (OLED) and a pixel driving circuit comprising a driving transistor (Tdr) adapted to drive the organic light emitting diode (OLED), the driving transistor (Tdr) having a characteristic that changes when the driving transistor (Tdr) extends or shrinks, each pixel being connected to one of the plurality of data lines (DL1,...DLd), one of the plurality of gate lines (GL1,..., GLg) and one of the plurality of sensing lines (SL1,...SLk); a data driver configured to drive the plurality of data lines (DL1,..., DLd); a gate driver configured to drive the plurality of gate lines (GL1,..., GLg); the data driver configured to supply sensing data voltages during a blank period of one frame period and to output data voltages corresponding to video data during a display period of the one frame; a sensing unit connected to the plurality of sensing lines and configured to determine a current flowing through the driving transistor (Tdr) and determine sensed data (Sdata) accordingly during the blank period and to output sensed data (Sdata) such that pieces of the sensed data (Sdata) vary according to a degree to which the driving transistor (Tdr) extends or shrinks; and a controller configured to determine a characteristic change of the driving transistor (Tdr) based on the sensed data (Sdata) transferred from the sensing unit and to determine whether the panel is bent based on the characteristic change, wherein the characteristic change of the driving transistor (Tdr) consists in a change in mobility or a threshold voltage.