Miles [1] original theory of wind generation by shear flows is extended to nonlinear waves. Two forms of nonlinear waves considered are: (i) the second-order Stokes wave where interaction between the first and the second harmonics had been neglected; and (ii) where the wave-wave interactions between harmonics had been taken into the account. The results in (i) show remarkable similarity to that of Miles but the results obtained for the case (ii) are considerably different compared with the quasi-linear theory of wave generation by shear flows [1]. The theory outlined here, estimates the magnitude of energy-transfer from wind to separate harmonics of nonlinear surface waves. This is important when considering wave groups that consist of several interacting harmonics. The results here also indicate that the initial wind speed required to grow nonlinear interacting waves is higher, but waves cannot grow indefinitely and the already generated waves begin to decay. However, at higher values of  the initial waves that are now decaying begin to grow. But due to harmonic interactions, the minimum wind speed required to grow these waves would be lower than that required for their initial growth. It is also shown that critical height rises by a factor of nearly 2 for interacting waves compared with that of non-interacting waves. This indicates that the critical height is no longer confined in the inner surface layer but reaches to the middle layer. The results of this investigation show that the wave steepness contributes in rising the critical height but the major contributing factor is the wave-wave interaction between the two harmonics of Stokes waves.

air-sea interactions, nonlinear waves, Stokes waves.