Abstract: An investigation on two aircrafts in formation flight was conducted in 1m unsteady wind tunnel. The leading aircraft was supported by a tail sting which was jointed to a vertical vane rod mounted on a frame of axes to achieve different relation positions accurately (including longitudinal spacing, lateral spacing and vertical spacing). The following aircraft was supported by primary support system of the wind tunnel to change its angle of attack. The aerodynamic forces and moments were measured by an inner six-component strain-gage balance, and the interaction flow structures were measured by a PIV system. Two sets of models were involved. One was the simple wing-body configuration and the other was flying wing configuration. Results show that lift-to-drag ratio of the following aircraft changes remarkably as the wingtip vortex of the leading aircraft approaches the following aircraft's wing, and when the vortex is close to the following aircraft's wingtip, the following aircraft can obtain the maximum lift-to-drag ratio. The following aircraft's aerodynamic characteristics change remarkably as the leading aircraft's angle of attack increases. However, lift-to-drag ratio of the following aircraft changes little when its angle of attack is bigger than 8° even with the presence of the leading aircraft.