Abstract: 【Objective】 Modeling dynamics of dry matter partitioning among unit organs (leaf, sheath and panicle) in rice is an important task for realizing virtual and digital plant growth. During the past years, great progress has been made on the dynamics of dry matter partitioning among leaves, stems and panicles of rice, but dry matter partitioning on unit organs of plant is seldom addressed. The present study was conducted to develop a simulation model on dry matter partitioning dynamics for various unit organs of rice in relation to GDD. 【Method】Four experiments were carried out involving different nitrogen rates and water regimes with four rice cultivars, and time-course observations were made on dry matter changes for different unit organs on stem and tillers. 【Result】The results revealed that the dynamic partitioning index of unit organ could be characterized by linear equation and exponential equation, respectively for leaf, and for sheath and panicle in relation to accumulated GDD. The change process of the maximum partitioning index for unit organ could be described with exponential equation (unit leaf and unit sheath) and quadratic equation (unit panicle) in relation to different leaf, sheath and panicle positions on main stem and tillers. In addition, the effects of nitrogen and water conditions on dry weight and partitioning index for unit organs were quantified through the effectiveness values of nitrogen concentration and water content in unit organs. The model was validated with the independent field experiment data involving different rice cultivars and nitrogen rates. The average RMSEs between the simulated and observed values for dry weights of unit organs on main stem and tillers under varied nitrogen and water levels with different cultivars were 0.93, 0.81, 6.8 kg/hm2 for leaf, sheath, and panicle, respectively. 【Conclusion】The results indicated that the present model had a good performance in predicting the dynamic changes in dry weight and partitioning index for different unit organs in rice under different growth conditions, and thus laid a foundation for further constructing digital and visual organ growth system in rice.