Santa Rosa Plum trees, 15 year-old were treated at pre and full bloom with auxin transport inhibitor (NPA) and Dormex ( hydrogen cyanamide) as well as their combination. The effects of different treatments on fruit setting, development and abscission in relation to endogenous phytohormone levels (auxin, gibberellins, abscisic acid) were studied either in fruit flesh or in stone seeds. Furthermore, auxin transport out of plum fruit during different developmental stages were determined. The results showed that, all treatments increased flower, as well fruit set  percentages and lowered abscission ratio concomitant with high fruit yield against untreated trees. The combined effect of NPA and dormex achieved the best yield. The average fruit weights reached 40, 38, 25, 24.5 grams by NPA (500 ppm); NPA (250 ppm); Dormex and dormex + NPA combination treatments respectively. Regarding endogenous hormone levels; high auxin level was detected in fruit flesh as compared to stone seed either by NPA application or NPA + Dormex combination. In contrast dormex application and untreated trees exerted low level of auxin in fruit flesh and high auxin level in stone seed. Similar trend for gibberellin levels were also detected either in fruit flesh or in stone seed. Whereas, high ABA level was found in fruit flesh and stone seed of untreated and treated trees with Dormex. The role of auxin transport out of plum fruit in regulation fruit setting, abscission and fruit development was discussed.