his study was carried out during two successive seasons (2015/016 and 2016/017) on Washington Navel orange trees grown in sandy–loamy soil under flooding irrigation system with Nile water in a private orchard at Qalub district, El-Qalubia Governorate, Egypt. This experiment studied the effect of spraying trees with diatoms at 0.5, 0.75 and1.00 kg or GA₃ at 25 ppm during 25 %, 75 % or twice 25 & 75% of full-bloom on inflorescences types, fruit set %, leaf pigments, leaf minerals content and  tree yield efficiency as fruit number & weight  and  fruit quality. Results showed that spraying of diatoms (0.5kg / 600L) on Navel orange trees at 75 % of full-bloom significantly increased leaf pigments contents. Spraying  diatoms  once at 25% or twice  25 & 75% % of full- bloom at 1.00 kg/600L improved with significant effect : fruit set % , leaf minerals content and  tree yield efficiency as fruit number & weight  and  fruit quality ( fruit size , fruit  hardness ) and (TSS %, TA % , TSS/ acid ratio and V. C content). Also, spraying of GA₃ at 25 ppm during 75% of full bloom statistically increased leaf iron, juice V. C and fruit hardness. Moreover, both diatoms and GA₃ reduced fruit drop and weight of peel when compared to the control. On the other hand, diatoms or GA₃ treatments had insignificant effect on leaf potassium, Chl. b, total carotenoids, zinc and number of both leafy and leafless inflorescences.