Balance wheel is commonly used in precision instruments. Its flatness plays an important role in the precision and accuracy of the whole instrument. Traditionally inspection of balance wheel is done manually. This study raises a flatness inspection system for balance wheel based on machine vision. First, the image of balance wheel is calibrated. Second, its sub-pixel edge information is extracted and boundary information is accurately calculated through projection mapping. The system supports continuous sampling and calculation and takes the dynamic range of the edge of the balance wheel as acceptability criteria of parts. The result of the test shows that this method has a satisfied real-time property and high accuracy, so it can meet industrial inspection needs.