The present study was carried out on fruitful Washington navel orange trees budded on sour orange rootstock planted on clay loamy soil with flood irrigation in El- Qalyubia  governorate, Egypt during two successive seasons 2020 and 2021 in a simple experiment with a complete randomized block design aimed to study using nano-potassium fertilizer compared with two conventional fertilizers potassium sources (Potassium nitrate and potassium humate) as foliar application with two levels each, three times/season on yield and fruit quality of “Washington" navel orange as well as their impact on fruits protein characteristics. Obtained results revealed that yield, fruit quality (physical and chemical) properties, and leaf nutrient contents (N, P, and K) responded differently to varied treatments. Potassium nitrate at 0.2% treatment increased fruit weight, juice weight, fruit total soluble solids, TSS/acid ratio, (lowest juice acidity %) and significantly displayed the reddish-orange fruit rind color. Application of 0.4% potassium nanoparticles (K-NPs) significantly enhanced the average yield, yield efficiency (kg/m3), and juice percentage, nevertheless unfortunately, this also revealed unsuspected variations in the genetic composition of eaten part of the fruit (whole segments) proteins compared to other treatments. Potassium humate treatment at 1% gave the highest V.C juice content.