Castor (Ricinus communis L.) is an industrial non edible oilseed adapted to drier areas. An experiment was conducted in the Central Rift Valley of Ethiopia to determine optimum plant population of castor variety Hiruy. Four plant (50, 60, 70 and 80 cm) and four row spacing (60,80,100 and 120 cm) were arranged in factorial combination and RCBD design in three replications. The experiment was carried out for two years at Melkassa and Arsi Negelle in the Central Rift Valley of Ethiopia. The result showed that plant population has no effect on the oil content of Castor. Plant spacing of 60 cm and row spacing of 80 cm resulted in longer main raceme, heavier seed weight and higher seed yield. Therefore, a plant population of about 20 833 plants per hectare or plant and
row spacing of 60 x 80 cm can be used for castor production in the Central Rift Valley of Ethiopia for an upper branching variety such as Hiruy. Length of main raceme, capsules per plant and seed weight are the main yield components in castor.
