Theoretical and testing calculation formulas of fractal dimension were introduced. The calculation method about box-counting dimension based on image processing was analyzed. Program about dimension and image processing based on Matlab were designed. Six weeds(2 of monocotyledon and 4 of dicotyledon) were picked out randomly and fractal dimensions of the six weed leaves were calculated using the program. Research results show that the calculation method for fractal dimension presented in this paper is reliable. Research results validate that weed leaf has fractal feature, and also prove that weed can be discriminated depending on feature parameters of fractal dimension because the fractal dimensions of varieties of weed are different apparently.