Abstract: To reveal the energy saving mechanism in fish schoolswimming, numerical simulation of the basic unit of fish school with threefish is presented. An adaptive mesh projection method is used to solve the2D incompressible Navier-Stokes equations, and the moving boundaries handledwith the immersed boundary method. The direction of swimming is controlledby the oscillating of fish head. The results show that three fish are ableto swim with the same speed and different frequencies. When the middle fishfalls behind about 0.4 times of body length, its swimming frequency is only54% of that of the other two fish, to demonstrate that the ``channelingeffect'' is remarkable. Therefore, to save energy, the fish in shoal shouldbe arrayed closely.