Eleven species of Thai medicinal plants, namely Rhinacanthus nasutus (L.) Kurz, Clitoria ternatea L., Mammea siamensis Kosterm., Centella asiatica (L.) Urban, Thunbergia lauriflolia Linn., Piper sarmentosum Roxb., Hibiscus sabdariffa L., Moringa oleifera Lam., Tinospora tuberculata Beume, Tiliacora triandra (Colebr.) Diels. and Amomum krervanh Pierre ex. Gagnep. were evaluated their allelopathic potentials against cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.) and crabgrass (Digitaria sanguginalis L.). The aqueous methanol extracts of these medicinal plants had inhibitory activity on all test plant species with different inhibition values. The aqueous methanol extracts of H. sabdariffa showed the highest inhibitory effect on cress and alfalfa seedlings. The extract obtained from P. sarmentosum, R. nasutus and T. tuberculata possessed the highest allelopathic potential on lettuce, timothy and crabgrass seedlings, respectively. Inhibitory effects of these medicinal plants were dependent on test plant species. The variation may result, in part, from the different test plant species with different sensitivity to allelochemicals. However, four medicinal plants H. sabdariffa, P. sarmentosum, R. nasutus and T. tuberculata possessed high allelopathic potential and may be good candidates for isolation and identification of allelochemicals. It would also be interesting to evaluate the implication on these evaluation results under field conditions.