Prediction of weed emergence timing would help minimize crop yield loss and reducing herbicide use by optimization of the timing of weed control. Therefore, seedling emergence evaluation was conducted in different irrigation levels (40, 60, 80, 100 percent of crop water requirement and furrow irrigation) to develop a thermal time seedling emergence model for Amaranthus retroflexus L. in sunflower field. Weed seedling emergence was monitored in plots from late-May through mid-September during growth season. Cumulative seedling emergence for A. retroflexus was described against thermal time using the Gompertz and Weibull functions. The Weibull model predicted seedling emergence of this species in different irrigation level with reasonable accuracy. The RMSE values of the model were ranged between 4.4– 9.7 percent showing that the predictions agreed closely with the observed seedling emergence. This species showed different patterns of emergence and thermal-time requirement for the onset of emergence in different irrigation levels. However, at 56 thermal-time unit, emergences occurred in different irrigation levels. In 80 and 100 percent of crop water requirement, there was an early peak of emergence followed by lower emergence later in the season. At thermal-time unit of 288, 90% of A.retroflexus seeds emerged in the furrow irrigation, suggested early emergence pattern of A.retroflexus in this irrigation system, while, seeds required 381- 413 thermal-time unit for 90% emergence in the tape system, indicating a late emergence pattern in tape system. Delay in seedling emergence could decrease weed competition ability against sunflower; however, late emergence could allow this species to escape weed control practices because most weed management strategies (e.g. cultivation and herbicide applications) are implemented early in the season.