The objectives of this research were to determine the inhibitory effects of wheat and weeds by soil application and different extractions of downy brome (Bromus tectorum, DB) and Italian ryegrass (Lolium multiflorum Lam., IRG) shoot and roots, and to identify inhibition substances by fermentation extraction of IRG shoot and roots. Shoot fresh weight (SFW) of two wheat cultivars, Stephens and Tubbs 06, was reduced 28 – 53% and 53 – 55% by DB residues grown at 25, 35, and 45 d after seeding (DAS) in sandy loam soil under greenhouse conditions, respectively, compared with the control treatment. SFW of wheat cultivars Stephens and Tubbs 06 was reduced 30 – 48% and 34 – 45% by IRG residues grown at 25, 35, and 45 DAS in sandy loam soil under greenhouse conditions, respectively, compared with those of the control. In soil application (150 g m^-2 or 300 g m^-2) of DB and IRG roots grown at 35 DAS, SFW of both wheat cultivars was reduced 33 – 52% compared with the control. However, the SFW of both wheat cultivars upon soil application of IRG and DB shoots was increased by increasing amounts of application. Common lambsquarters (Chenopodium album L.), pigweed (Amaranthus alus L.), spiny sowthistle (Sonchus oleraceus L.), white clover (Trifolium repens), barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.), and large crabgrass (Digitaria ciliaris) were inhibited 23 – 95% and 25 – 80% by soil application of IRG and DB roots, respectively, compared with the control. However, weed growth inhibition was less affected by these treatments. The shoot and root fresh weights of both wheat cultivars were inhibited by water extracts of IRG and DB shoots at 0.5%, 1%, 2.5%, 5%, and 10% concentrations, but not by water extracts of IRG and DB roots. Reduction of shoot and root fresh weight in both wheat cultivars was observed more in fermentation extracts of IRG shoots and roots than in water extracts of IRG roots. Phenol compounds hydrocinnamic acid, caffeic acid, p-coumaric acid, and ferulic acid were confirmed in fermentation extraction of DB and IRG shoot and roots by high performance liquid chromatography (HPLC); the contents of phenol compounds were greater in DB and IRG shoots than in roots. Shoot and root weight of both wheat cultivars was inhibited 42 – 69% by 0.5, 1, and 3 mM treatments of phenol compounds p-coumaric acid, ferulic acid, and caffeic acid. Therefore, retarded growth of wheat and weed may have been caused by the phenol compounds of DB and IRG.