The viscous effect on the high-speed impulsive (HSI) noise for a helicopter rotor has been investigated by using a combined method of computational fluid dynamics (CFD) techniques with the extended Kirchhoff's equation. The pressure distributions around a rotor blade are obtained by Euler and Navier-Stokes equations to capture a precise shock behavior.The Kirchhoff's equation extended to a moving surface is applied to calculate the acoustic pressures at the observer point. In this calculation, the pressures and pressure gradients on the Kirchhoff surface, in which all the acoustic sources are enclosed, are obtained by using CFD techniques. In order to estimate the effect of the viscosity on the acoustic pressures, two types of turbulence model in Navier-Stokes calculations are used. One is Baldwin-Lomax model and the other is Coakley's q - ω model.The HSI noise of a non-lifting hovering rotor is calculated by using the present method, and the reasonable correlation between calculated and experimental results are obtained. The effect of the viscosity on the acoustic signature showed slight difference after the delocalization occurred on the rotor blade. On the other hand, there is little difference on the acoustic signature before the delocalization.