Root knot nematodes Meloidogyne incognita are responsible for heavy economic losses to many agricultural crops and considered the most difficult crop pest to control. Recently attention on environmental and food safety in addition to human health has led to increase the limitations on using chemical nematocides and searching for environmental safe natural nematocides. Saponins extracted from Medicago sativa L., alfalfa was used to control the infection of tomato seedlings with root knot nematode Meloidogyne incognita. The results indicated that saponin treatments led to significant reduction in the numbers of larva of root knot nematodes in tomato roots and in soil. The highest nematode inhibition was observed in the treatment of 100% of saponin crude extracts. The observed reduction of the number of nematode larva in tomato roots and in soil were found to be correlated with the decline of cholesterol level in root knot nematode eggs which is due to saponins from Medicago sativa, in a concentration dependent manner. Also saponin treatments showed a general improvement in plant growth and performance.