The present study was carried out at Suleyman Demirel University research farm in Isparta during 2008 and 2009 crop seasons to determine the effects of leonardite (54.5% organic matter) on yield and quality of potato. Four leonardite doses (0, 200, 400, 600 kg ha(-1)) and four potato cultivars (Van Gogh, Milva, Lady Olympia, Agata) were used in the study. A total of seven traits (plant height, tuber number per plant, marketable tuber yield, total tuber yield, protein content, vitamin C content and specific gravity) were measured. Significant differences were detected between control and leonardite applications for all the traits studied. Statistically significant differences were not found between the leonardite doses for plant height and specific gravity. There was no significant difference between 400 and 600 kg ha(-1) leornardite application for tuber number per plant, marketable tuber and total tuber yield, protein and vitamin C contents. Leonardite applications increased number of tubers per plant by 22%, marketable tuber yield by 38% and total tuber yield by 15% compared with the control. These results suggest that 400 kg ha(-1) leonardite combination to the standard fertilization will be sufficient to obtain adequate yield and quality tubers in potatoes.