Abstract: Precision agriculture necessitates the utilization of less quantity of agricultural chemical. By using analysis image, weeds or the proportions of crop and weed leaves can be located in field, but overlapping of crop and weed leaves makes it difficult to distinguish them in color image. In this paper, two different methods are suggested to distinguish onion and weed leaves in two different cases. One method is for non overlapping of onion and weed leaves, the other is for the case with some overlapping. The former method occupies a little of calculating time, the latter method needs a great deal of calculation. A mode of merging regions with two stages and with different attributes is also suggested. The different attributes include: the average of color components of pixels, the rates of frontiers and the shape factor of region.