Crop models and decision support system can be useful tools for researchers, teachers, scientists, extension educators, policy makers and planners to help or support the application and evaluation of sustainable and long term alternative management practices. Sunflower is an important oilseed crop that has the potential to bridge the gap between the consumption and domestic production of edible oil in developing countries such as Pakistan. Limited research has been conducted on the application of the OILCROP-SUN model for the simulation of growth and development of sunflower under different environmental conditions, especially in developing countries. Field experiments were conducted in three environments to study the impact of nitrogen (N) fertilizer on grain yield and quality. The objective of the present study was to evaluate the performance of the OILCROP-SUN model and to determine the effect of N on sunflower for three contrasting environments. The results showed that OILCROP-SUN model simulated well for growth, development and yield of sunflower hybrids in these three contrasting environments and under different management practices that included several genotypes and different nitrogen fertilizer application rates. The d values (index of agreement) for leaf area index (LAI) ranged from 88 to 97 under arid, 78 to 85 under semi- arid and 71 to 81 under sub- humid conditions. Similarly, d values for total dry matter (TDM) ranged between 90 and 94 under arid, 81 and 88 under semi-arid and 90 and 96 under sub-humid conditions, respectively while the percentage difference for grain yield ranged from 8.5 to 21% for different ecological conditions. The model also simulated LAI, TDM, grain yield and oil quality well under different environmental conditions of Pakistan and further studies should focus on application of the OILCROP-SUN model for variable agro-climatic regions where sunflower is an important crop.