In 2003 and 2004, a study was conducted to determine the effect of different water stress levels (0.0, 0.25, 0.50, 0.75 and 1.0, which was full irrigation) imposed on soybean and to compare with CROPGRO-Soybean model simulations under semi-arid climatic conditions of Harran plain, Sanliurfa, Turkey. Soybean plots were 5 m long with five plant rows of crop with 75 cm row and 4-5 cm plant spacing. Irrigation system used to irrigate soybean plots was a drip irrigation system with 20 cm-spaced emitters with flow rate of 4 L h(-1). Soybean growth simulation model used in this study was part of DSSAT v. 3.5 version (Decision Support System for Agrotechnology Transfer). In 2003 and 2004 full irrigated (1100) plots had crop ET values of 574 and 619 mm, when simulated ones for the same treatments ranged from 595 to 576 mm, respectively. Measured yields ranged from 322 to 3684 kg ha(-1) and 267 to 3952 kg ha-1 for the years of 2003 and 2004, respectively, while simulated results for the same years varied from 616 to 3675 kg ha-1 and 543 and 3690 kg ha(-1). Analysis of applied irrigation amounts and observed yields indicated a strong relationship (P < 0.01). Furthermore, regression analyses of yield data indicated a statistically significant (P < 0.05) relationship between measured and simulated yields with R-2 values of 0.94 and 0.88 for 2003 and 2004, respectively. overall, reduction in irrigation water amounts lowered yield in both years and similar results were obtained from simulations. In general, CROPGRO-Soybean model satisfactorily simulated soybean yields and yield components. (C) 2007 Elsevier B.V. All rights reserved.