Wheat (Triticum aestivum L. cv. Mulchmugi) and barley (Hordeum vulgare L. cv. Minorimugi) were introduced in watermelon production for weed control, not seed production. To examine the weed control-effect of wheat and barley between rows of a watermelon field, they were sown in October, 1993, with the seeding rate of 4kg/10a. Elongated culms were mowed early in June, 1994, and turned into straw mulch. Light intensity (photon flux density) on the surface of the ground was suppressed by straw mulch of wheat and barley. Suppression of light intensity in wheat straw mulch 1 month after mowing was greater than that of barley because of the large biomass and its toughness. Matured seeds of the mowed culm germinated late in June, and new leaves developed above ground. Suppression of light intensity by new leaves of barley plants was more than that in wheat, and photon flux density on the surface of the ground was reduced to 30% of solar radiation. Weed quantity between rows was effectively reduced, especially by wheat straw mulch. Ground surface cover with triticale (x Triticosecale Wittmack cv. Ryeducks) was sparse because of fewer numbers of culms. Triticale was not suitable for the plant material of straw mulch in watermelon cultivation since the new shoots developed from the stubble after mowing and lifted the watermelon vines up. Intercropping system decreased the working hours for laying of straws and weedingin the field, and consequently led to labor saving.