This study was carried out during three consecutive (2016/019) experimental seasons on 15- year-old fruitful Valencia orange trees (Citrus sinensis L. Osbeck) Olinda cv., budded on Volkamer lemon rootstock, grown in reclaimed soil at two sites (South of El Tahreer& West of Al Nubaria regions) under drip irrigation system (Nile water) and subjected to the same cultural practices. This work aim to study the role of Boron in fruitful Valencia orange trees during the 1st season through an exploratory study at the 1st season ( phase "I"), then using foliar application of different Boron sources (Boric acid at 1% ; Boron / Molybdenum compound at 1% ; Mulase “CM" at 3ml / L., and Boric acid at 1% + Mulase “ CM" at 3ml / L. and control treatment, at the suitable stage to compensate Boron deficiency during the other two seasons (phase “II"). Results indicated that, there is a fluctuation in Leaf Boron content(ppm) from one site to another during the phase “I" of the study for both sites, whereas, at the 1st site seemed to be constant during the most samples (from 10 to 20 ppm) and slightly reduction ( 50 ppm) for October sample. On the other hand, leaf Boron content under “Site II" conditions was higher (from 30 to 40 ppm) than the site"I" (from 10 to 20 ppm).Moreover, it has increased during February (up 30 ppm), then sharply reduced at the March sample (