In order to realize the automatic measurement of the invar rod scale errors, an automatic measurement system of invar rods based on image processing is proposed. The scale line under test is imaged by a microscope. The scale line image is acquired and transmitted to the computer by a digital CCD video camera. Digital image processing technology is used to extract the scale line edges. Then the scale midline position coordinate originated from the first-measured scale line can be estimated automatically. The scale error is calculated out. The uncertainty from measurement repeatability is about 0.79 μm and the extended uncertainty in the measurement is about 3.9 μm which excels the 6 μm prescribed in the national standard. The whole system integrates optics, mechanics, electrics and computer together and can be operated conveniently with high accuracy.