The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows: the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. Furthermore, based on the principle of minimum dissipation of mechanical energy together with conservation equa-tions, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid flow in the fixed flow domain. Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presented. The optimized velocity dis-tributor nearby the fork, which was designed based on the principle of minimum dissipation of me-chanical energy, may reduce the drag of duct flow with two parallel branches. The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows: the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. of mechanical energy together with conservation equa-tions, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presente d. The optimized velocity dis-tributor nearby the fork, which was designed based on the principle of minimum dissipation of me-chanical energy, may reduce the drag of duct flow with two parallel branches.