In order to assess the effect of soil water deficit (SWD) at different growth stages, on yield and quality of processing tomato under deficit irrigation in the sub humid climate, an open-field experiment was carried out in 2010 and 2011 in the experimental site of the Mustafakemalpasa Vocational School, Uludag University, Turkey. A randomized complete block design with three replications was used. Four known growth stages the plant (V: vegetative, F: flowering, Y: yield formation and R: ripening) were considered and a total 15 irrigation treatments were applied (VFYR, FYR, VFY, VFR, VYR, VF, VY, VR, FY, FR, YR, V, F, Y and R). The effect of SWD at any stage of growth on fruit yield suitable for processing, soluble solid content, dry matter content, pH, total sugar, total acidity, lycopene, carotene and vitamin C, were evaluated. The results revealed that fruit yield and all quality parameters considered in this study were significantly affected by SWD due to omitted irrigation during the sensitive flowering and yield formation. Water deficits occurring during ripening stage increased values of soluble solid content, dry matter content, total sugar, total acidity, lycopene and total carotene. Highest yields were observed in the fully irrigated control (VFYR) and the treatment which allowed SWD during the vegetative stage (FYR). Even irrigations omission during one of the sensitive growth stages, caused up to a 10% loss of fruit yield. Much greater losses of 52-64% could be expected as a result of prolonged SWD during flowering and yield formation stages. In particular, relatively to the sub-humid areas of tomato cultivation, the best compromise between quality and quantity of the processing tomato fruit was achieved with omitted irrigations during only ripening stage (VFY treatment) although yield loss was about 5%.