Abstract: Based on the idea that organisms always tend to make full use of materials,a mathematical model of plant bifurcation is set up on some experimental results. This model suggests that,when the relation “parallelogram rule” exists between main stem and sub-branches,the total volume of branches is minimal.This model reveals that plant branching not only meets the law of mechanical equilibrium,but also is important in the sense of evolution.A concept,diverge degree,is put forward on the measured data,which reveals that the structure of branches is associated with interaction of different branches in a plant.An analysis supports that bifurcations of plants still have optimal forms even their branching angles are affected by other factors such as space effect,etc.Because of the variety of the model,bifurcations have varied forms.The model does well in combining bifurcation angles and section areas of branches and it will improve the way by which plants are simulated by computers.