Abstract: For a helicopter in forward flight, it is still unknown whether the lateral induced velocity on the advancing side of the rotor plane is. larger than that on the retreating side or vice versa. In order to clarify the essential feature of the lateral induced velocity distribution of the helicopter ro tor, the contributions of circulations 0 and 1s to the sine component of the induced velocity are analysed in this paper, based on the classical vortex theory. Then, using the flapping condition of the rotor blade, a critical radius position is found. On the outboard blade（outside of the critical radius position）, uls- negative, the induced velocity on the advancing side is amaller than that on the retreating side whereas on the inboard blade （inside of the critical radius position）, vls is positive, the indeced velocity on the advancing side is larger than that on the retreating side. This conclusion is verified by experiments.