This study was carried out during four successive seasons of 2006, 2007, 2008 and 2009 on Picual olive trees 12 years old growing planting  in a sandy soil spacing at a 5x6 m. at a private farm on Cairo Alex. Road to investigate the effect of different thinning chemicals (Ethrel at100ppm,200ppm, 300ppm, NAA at100ppm,150ppm and urea at 2%,3%)on biennial bearing index, fruit quality and vegetative growth of Picual olive trees in both "On" and "Off" years. The obtained results indicated that all tested treatments reduced alternate bearing severity as compared with the control during the four seasons of study through thinning fruits of on-years (2006&2008) and increasing flowering and fruiting in off-years(2007&2009). In addition, the treatment produced fruits with good quality sinceit increased fruit weight, flesh/stone ratio, fruit dimensions and oil content, The most effective treatments were, ether at300 ppm and NAA at. 150 ppm. Besides, vegetative growth parameters , shoot length and thickness, number of leaves per shoot and number of internodes per shoot were promoted.  The experiment led to the conclusion that fruit thinning of olive with ethrel at 300 ppm or NAA at 150 ppm in on-years of biennial bearing cycle reduce alternate bearing severity and improved fruit quality in both "On" and "Off" years