A field experiment was conducted on ten years old "Peyuan" ber (Ziziphus mauritiana Lam.) cultivar during 2005 and 2006 seasons. The trees were grown in loamy sand soil at the Agricultural Research and Experiment Station (Dirab), College of Food and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia. The aim of the present study was to investigate the effect of potassium fertilization at rates of 0, 500, 750, 1000 and 1250 g K₂O /tree / year on yield, fruit quality and leaf elemental contents. The obtained results for both seasons revealed that K treatments significantly increased fruit yield. Potassium fertilization significantly increased the average fruit weight, size, length, diameter, flesh weight percentage over the control in 2005 and 2006 seasons. The highest values were obtained with 750 g K₂O /tree/year in both seasons. The control trees gave the lowest percentage of fruit grade 1 compared to K treatments in both seasons. However, 750 g K₂O /tree/year treatment had a highest percentage of fruit grade 1 (44.53 and 58.03% in both seasons, receptivity) and values of fruit flesh firmness. Treatments of 1000 and 1250 g K₂O /tree/year in both seasons, gave the highest values of fruit V.C compared to the control, 500 and 750 g K₂O /tree. The acidity percentage in the fruit juice decreased significantly among the potassium treatments in both seasons. The percentage of total soluble solids (TSS), reducing, non–reducing and total sugars followed nearly similar trends as they were affected significantly by potassium treatments in both seasons. Thus, it is obvious that the highest mentioned parameters were found in fruits collected from 750 and 1000 g K₂O /tree/year treatments in both seasons. On the other hand, the least values always resulted from control (zero g K₂O /tree/year) treatment in both seasons. Generally, applying K at 750 g K₂O /tree/year improved yield, fruit quality and nutrient uptake. Thus, such treatment could be the promising treatment for "Peyuan" ber cultivar grown under similar conditions.