Simulation models are very useful for prediction of rice phonological stages and crop adaptation to different agroechological systems. The objective of this study was to calibrate and validate the ORYZA2000 model under optimum production conditions for simulatinig and quantifying of the phonological development and crop duration of three rice cultivars as influenced by different seedling ages to improve crop management in rice growing areas of Iran. To calibrate crop parameters and validate of ORYZA2000, field experiments were carried out in Haraz Center for Development and Extension of Technology, Iran, during the 2008 and 2009 growing seasons. The experiment was conducted as a split plot arrangement in randomized complete block design with 3 replications. Main plots were three rice cultivars, Tarom, Fajr and Ghaem1, and Sub plots were three seedling ages consisted of seedlings 17, 24 and 33 days old. Observed crop data set were collected from the first year field trail for crop parameter estimation in ORYZA2000 and those from the second year for model validation. The ORYZA2000 model, with range of absolute RMSE values between 3-4 days, had high accuracy for prediction of different phonological stages of rice cultivars. Considering appropriate ability of ORYZA2000 model in simulation of phonological stages of rice cultivars, it can be used as an appropriate tool for programming of paddy field management and appropriate decision support system for adaptation trials of improved and introduced rice cultivars in different agro- climatic regions.