Soybean (Glycine max L.) crops were grown in 2003 and 2004 to determine effect of different water stress levels on growth, some physiological variables and seed composition under semi-arid climatic conditions of Harran plain, Sanhurfa, Turkey. Irrigation water treatments included 0 (I-0), 25 (I-25), 50 (I-50), 75 (I-75), and 100% (I-100) of full irrigation amounts with a 4 day irrigation interval. Water stress resulted in reduced vegetative growth, leaf relative water content and leaf chlorophyll content. In 2003 and 2004 seed yields ranged from 322 (I-0) to 3,684 kg ha(-1) (I-100), and from 267 (I-0) to 3,952 kg ha(-1) (I-100), respectively. Compared to 1,00, the two-year average seed yield reduction for I-0, I-25, I-50 and I-75 were 92, 76, 50 and 25%, respectively. Water stress significantly decreased fatty acid content of C18:0, C18:1, C18:2, and C18:3 but not C16:0. I-100 produced the highest protein value (39.60% in 2003 and 39.00% in 2004), while I-0 yielded the lowest values (35.41% in 2003 and 36.11% in 2004). A negative correlation between evapotranspiration (ET) and oil content and ET and protein content in both years was determined. Leaf chlorophyll content, leaf relative water content, leaf area index, number of branch per plant, number of node per plant and plant diameter correlated positively with seed protein content and negatively with oil content.