1. In this ecological studies, the close relations among the crabgrass and several crops were observed in the upland field. 2. Branched ears of the crabgrass (dominant species) were correlated to numbers of its seed. 3. Falled seeds of this weed were found 218.2 in ten square centimeters after the sweet-potatoes were harvested. On the other hand, the seedlings of the crabgrass were reckoned 22.4 in average of twenty plots on which wheat plants were cultivated, and 33.3 on the none crop field. 4. Yields of the upland rice and the sweet-potato were badly influenced by the crabgrass. The wheat, however, was suffered little or nothing.