The allelopathic effect of medicinal plant, Piper sarmentosum Roxb., against 12 test plant species was evaluated. Four different concentrations of 0.01, 0.03, 0.1 and 0.3 g dry weight equivalent extract mL-1 were used in the study. The hypocotyl and root length were measured compared with control treatments. It was observed that the aqueous methanol extracts of P. sarmentosum plants inhibited all test plant species with different inhibition values. The variation may result, in part, from the different test plant species with different sensitivity to allelochemicals. The shoot and root growth of test plants were inhibited at the concentration greater than 0.03 g dry weight equivalent extract mL-1 and increasing the extract concentration increased inhibition. These results suggesting that P. sarmentosum may contain growth inhibitory substances and possess allelopathic activity. The concentrations required for 50% growth inhibition of test plants were 0.001-0.210 g dry weight equivalent extract mL-1 and alfalfa seedling were most sensitive to the extract. P. sarmentosum may be good candidate for isolation and identification of allelochemicals. The information obtained could be utilized in the development of bioherbicide for future weed management.