A line-width measurement device and a measurement method using the same are disclosed. The line-width measurement device has a platform, an image capturing device and a color-mixing light source device. The image capturing device captures an image of a pattern under measurement in a measurement area of the platform. The color-mixing light source device correspondingly provides illumination to the measurement area. The color-mixing light source device has a plurality of monochromatic light sources and adjusts the brightness scale of each monochromatic light source according to the matching rate of the pattern under measurement and a standard pattern to provide suitable color-mixed lights for illumination. Therefore, the present invention can provide a better measurement environment to further enhance accuracy of line-width measurement.