An algorithm of producing navigation line for robot in forestry environment based on machine vision was developed. By scanning an image, boundaries between the tree bottom and earth are recorded, and the central points between both tree bottom sides are gained and a straight line generated by least-square fitting to perform as the navigating line. A central line was obtained as the navigation guidance line. Matlab simulation results show the arithmetic efficiency in various forestry environments, in accordance with human vision recognition. The reliability and stability are demonstrated during batch processing. 