An organic light emitting diode (OLED) display device including a plurality of pixels to display images, each of the pixels including an OLED, a driving transistor connected to the OLED, and a switching transistor configured to supply data signals to the OLED, the device comprising: a sensing unit configured to sense first and second output voltages (Vsen1, Vsen2) output at a source of the driving transistor of each pixel respectively in response to applying first and second data voltages (V1, V2) to a gate of the driving transistor, the first and second data voltages (V1, V2) being greater than a threshold voltage (Vth) of the driving transistor; a compensation value calculator configured for each pixel to obtain a linear functional relationship between the first and second output voltages (Vsen1, Vsen2) and the first and second data voltages (V1, V2), to obtain a slope of a graph representing the linear functional relationship, to obtain a reference slope of a reference graph representing a linear functional relationship between first and second reference output voltages (V1out, V2out) and the first and second data voltages (V1, V2) on the driving transistor, wherein the first and second reference output voltages (V1out, V2out) are sensed in a product shipping step at the source of the driving transistor of the pixel in response to applying the first and second data voltages (V1, V2) to the gate of the driving transistor and stored in a memory, to obtain the change amount of the mobility of the driving transistor as the difference between the slope of the graph and the reference slope of the reference graph, to obtain an intercept of the graph on a x-axis, to obtain a reference intercept of the reference graph on the x-axis; and to obtain a change amount of the threshold voltage (Vth) of the driving transistor as the difference between the intercept and the reference intercept; and a data compensator configured to adjust the data signals based on the change amounts of mobility and the change amounts of the threshold voltage (Vth). a sensing unit configured to sense first and second output voltages (Vsen1, Vsen2) output at a source of the driving transistor of each pixel respectively in response to applying first and second data voltages (V1, V2) to a gate of the driving transistor, the first and second data voltages (V1, V2) being greater than a threshold voltage (Vth) of the driving transistor; a compensation value calculator configured for each pixel to obtain a linear functional relationship between the first and second output voltages (Vsen1, Vsen2) and the first and second data voltages (V1, V2), to obtain a slope of a graph representing the linear functional relationship, to obtain a reference slope of a reference graph representing a linear functional relationship between first and second reference output voltages (V1out, V2out) and the first and second data voltages (V1, V2) on the driving transistor, wherein the first and second reference output voltages (V1out, V2out) are sensed in a product shipping step at the source of the driving transistor of the pixel in response to applying the first and second data voltages (V1, V2) to the gate of the driving transistor and stored in a memory, to obtain the change amount of the mobility of the driving transistor as the difference between the slope of the graph and the reference slope of the reference graph, to obtain an intercept of the graph on a x-axis, to obtain a reference intercept of the reference graph on the x-axis; and to obtain a change amount of the threshold voltage (Vth) of the driving transistor as the difference between the intercept and the reference intercept; and a data compensator configured to adjust the data signals based on the change amounts of mobility and the change amounts of the threshold voltage (Vth).