This paper presents the blade vortex interaction (BVI) noise reduction study using the blade active control. The study consists of the following three steps. As the first step, the BVI noise for the conventional blade was measured using MultiPurpose Rotary Wing Test-rig (MPRT) in JAXA (Japan Aerospace Exploration Agency)[1]. It is important for the understanding BVI phenomena to know where the interaction between blade and vortex occurs. Sound pressure and blade surface pressure histories were obtained and the locations of the interaction were acquired. As the second step, prediction of BVI phenomena was tried using a noise prediction code. The simulation was done at the same condition of the BVI wind tunnel test and the comparison between numerical results and test data was done[2]. The quantative agreement between the test result and numerical one is still insufficient but interaction between the blade and vortex was able to be visualized. As the third step, active control rotor was developed and was evaluated the ability to reduce the BVI noise by the low speed wind tunnel test. Trailing edge flap is used to control the circulation of rotor blades to avoid strong interaction of blade and tip vortex. Active flap is controlled using piezoelectric actuators. The active rotor wind tunnel test was conducted in the same speed wind tunnel as the conventional rotor test. The effect of the active flap was rather small but further optimization of flap angle and location will improve noise reduction effect. Nomenclature C = Speed of sound CT = Rotor thrust coefficient e = Energy J = Jacobian M = Mach number n = Number of rotation of Main rotor P = Pressure t = Time u,v,w = x,y,zvelocity V = Flight velocity x,y,z = Spatial coordinates αs = Rotor shaft angle (positive nose up) θ = Blade collective angle μ = Advance ratio, V/ ΩR ξ,η,ζ = Computational coordinate ρ = Density τ = Nondimensional time ψ = Blade azimuth angle Introduction Helicopter is one of the most convenient transportation. They can take us almost everywhere if enough space exist for landing or sling off us. The noise problem, however, barred wide spread use of helicopters. Most of the commercial use helicopters flies near the town. The aeronautics research is reducing noise to make helicopters more acceptable to the public. Active blade control method has been applied to reduce blade noise since 1970’s. The effectiveness has been confirmed but the practical applications of such methods are not realized yet. The difficulty exists on the blade control method. Several methods have been developed for this thirty years. HHC(Higher Harmonic Control) and IBC (Individual Blade Control) are the famous blade control systems. HHC is the method to control a swash plate at high frequency to reduce vibration or rotor noise. IBC is the method to control each blade pitch angle individually. Active flap method can be called one sort of the IBC methods, however, conventional IBC methods use actuators between blade root and swash plate. High response and power are required for the IBC actuators. The merit of the active flap is that little force is required to control blade in comparison with conventional IBC method. The problem of the active flap method is developing the actuator, which is enough small to