Fruit thinning in apple crop plays a critical role to achieve marketable king size fruits with better fruit quality apples. The thinning experiments were conducted using naphthaleneacetic acid (NAA) at different concentrations (0, 10, 15 and 20 μl·l^-1) applied at three timings [full bloom (FBT), fruit (FT) and the combination of both (FBT + FT)] in order to evaluate the optimum level of thinner with best application timing on fruit quality attributes of cv. 'Granny Smith' fruits. Statistically significant differences were found regarding the effect of timing, concentration and combination of both, timing and concentrations, on firmness, soluble solids concentration (SSC) and titratable acidity (TA), while there were no significant differences with addition of timing for SSC/TA ratio, fruit mass and starch degradation level. The highest fruit mass (195.09 g) and TA (0.67%) was achieved using NAA at 20 μl·l^-1 in FT time while NAA at 15 μl·l^-1 resulted in highest fruit firmness (7.25 kg cm^-2), SSC (13.04° Brix) in FBT time. SSC/TA ratio was the highest (25.31) at FT time with the same NAA dosage. However, the highest starch degradation was achieved at FBT + FT time with 15 μl·l^-1. It can be concluded that application of 10 μl·l^-1 NAA at FBT + FT time is recommended to achieve best results since it favourably affected the most quality parameters.