In this study, effects of juglone and decomposed walnut leaf juice on corn (Zea mays var. amylacea), cucumber (Cucumis sativus L.), radish (Raphanus sativus L.), Onion (Allium cepa L.) and bread wheat (Triticum aestivum L.) seed germination percentage and post germination seedling growth were investigated. Decomposition was carried out by keeping the leaves in distilled water. Corn, cucumber, radish, Onion and bread wheat seeds were germinated in Petri dishes at room temperature. Seed germination and seedling elongation were determined at day 7. Juglone and decomposed walnut leaf juice significantly decreased cucumber and onion seed germination percentage. However, germination of corn and radish seed was not affected. Juglone and decomposed walnut leaf juice also significantly decreased root and stem elongation of cucumber and onion. Bread wheat was only affected by undiluted decomposed walnut leaf juice.