Abstract: Allelopathy refers to the chemical inhibition of growth of one species by another. With the objectives of understanding allelopathic effects of sixty seven traditional rice cultivars, a field experiment and laboratory experiments were carried out in 2011 Yala season at Faculty of Agriculture, University of Ruhuna, Mapalana, Sri Lanka. Rice cultivars were transplanted according to the completely randomized block design with four replicates. Each replicate consisted of three rows of rice plants and data were collected from the middle row. After three months of transplanting, plant height (cm) and number of tillers/plant, were evaluated in traditional rice cultivars and total dry matter weight of weeds in 0.093 m2 (1ft2) area encircled by the rice plant were evaluated in three replicates.