The aim of the present study was to evaluate the influence of potassium fertilization on tuber yield and starch content of potato tubers. The field trial focused on three potato cultivars: Muza, Oda i Orłan, which were cultivated on three mineral fertilization levels: N40P17, N80P34, N120P51. Potassium elevated rates were applied proportionally to N and P in the quantitative relation of: 0, 1, 2, 3. The tuber yield of cultivars were significantly different. The highest yield was noted for Orłan cultivar– 38.34 t ha-1, and significantly lower for Oda and Muza – respectively at 35.33 and 31.83 t ha-1. The yielding depended on climatic conditions during vegetation period. The yields noted for 1998 and 2000 were comparable (46.65 t and 40.05 t ha-1) but lower precipitations in 1999 and high temperatures during the second half of vegetation significantly reduced tuber yields to 22.21 t ha-1. Increasing NP fertilization improved the tuber yield, especially for the Muza and Orlan cultivars. The starch content depended on meteorological conditions too. The highest content of starch was noted for tubers harvested in 2000 – average 14.21%, lower in 1998 – 13.94%, and lowest in 1999 – 13.86%. The low concentration of tuber starch in 1999 caused lowest starch yield (3.08 t ha-1) compared with 1998 (6.50 t ha-1) and 2000 (5.69 t ha-1). The fertilization ranges from N40P17 to N120P51 significantly reduced tuber starch content of cultivars. Increasing doses of potassium used on the three NP fertilization levels caused a lower starch content in tubers of the estimated cultivars.