The present invention provides method and device for driving local dimming in a liquid crystal display device which enables adaptive application of a gradation roll-off according to an image characteristic. The method for driving local dimming in a liquid crystal display device includes the steps of determining a local dimming value of each light emitting block by analyzing a received image data light emitting block by light emitting block of a backlight unit, producing a pixel compensating coefficient on a light quantity change of each pixel by using the local dimming value of each light emitting block, producing a required gradient value by compensating the received image data by using the pixel compensating coefficient, and producing maximum required gradient values for one frame and an average value of the maximum required gradient values for one frame, determining a roll-off end point of a gradient roll-off section according to the maximum required gradient value, and determining a roll-off starting point of the gradient roll-off section according to the average of the maximum required gradient values, setting a gradient change curve of the gradient roll-off section by using the roll-off starting point and end point, and producing a gain value of each pixel from the gradient change curve, and forwarding an output gradient value by correcting the required gradient value by using the gain value of each pixel.