Objective: Biological control is useful in agricultural. Allelopathy plays an important factor in crop productivity. The effect of allelopathy is able to produce and release allelochemicals or phytochemicals to inhibit or stimulate seed germination, seeding growth, shoot and root growth of other crops. The aim of this study was to evaluate the inhibitory activity of Cleome viscose and Cleome spinosa extracts on onion guest roots length.Methods: The whole plants of C. viscose and C. spinosa were extracted with dichloromethane, ethyl acetate, methanol and distilled water, respectively. Each group of onion guests was sprayed at various concentrations as 100, 200, 400, 800 and 1600 parts per million for the treatment. The root growth was measured every day for 1 month.Results: At concentration of 1600 ppm, methanol crude extract of C. spinose showed the highest percent inhibitory activity value of 91.68. Ethyl acetate and methanol crude extracts of C. viscose showed strong inhibitory activity with percentage values of 90.17 and 90.90, respectively, at concentration of 1600 ppm. Moreover, the methanol extract of C. viscose and C. spinosa evaluated higher inhibitory activity than other solvent. However, distilled water crude extract of C. spinose showed weak inhibitory activity with the percentage value of 40.05 at concentration of 1600 ppm.Conclusions: In this study, the methanol crude extract of C. spinose showed potent inhibitory activity on root growth. Moreover, ethyl acetate and methanol crude extracts of C. viscose evaluated strong inhibitory activity. C. viscose and C. spinosa extracts possessed allelochemicals for postharvest biology and technology in plants.