Abstract: Several experiments were conducted in greenhouse to evaluate the effects of 10 mechanical oil concentrates on the efficacies of cyanazine and nicosulfuron for the control of barnyardgrass (Echinochloa crusgalli (L.) Beauv.) and velvetleaf (Abutilon theophrasti Medic.). Results showed that all the mechanical oil concentrates enhanced the activity differently with different viscosity. Of 10 adjuvants, the one with viscosity of 46 mm2/s (40℃) (MOC46) was most effective and that with 32 mm2/s (40℃) followed. The enhancement to cyanazine is better than to nicosulfuron. The order of enhancement of MOC46 on different herbicides was: atrazine and cyanazine＞atrazine+cyanazine and atrazine+nicosulfuron＞cyanazine+nicosulfuron＞nicosulfuron. All the treatments above, the order of safety to maize seedlings was: atrazine/nicosulfuron/atrazine+nicosulfuron＞cyanazine+atrazine and cyanazine+nicosulfuron＞cyanazine. Cyanazine with the addition of MOC46 had detrimental effects on maize seedlings.