Abstract A series of experiments were conducted in a super-wave flume (300 m×5 m×5.2 m) to examine the low-frequency motion induced by waves with different incident steepness, sloping gradients and normalized frequency (sideband space). Two kinds of waves including initial uniform wave train and modulated wave train (one carrier with a pair of sidebands) are utilized for incident wave conditions. From the experimental results, it is found that for a given slope gradient the infra-gravity wave component decreases as wave nonlinearity increases and frequency downshift is a predominant factor. Furthermore, the magnitude of low-frequency component decreases with slope gradients for a given initial wave condition. In addition, the maximum value of low-frequency motion is found to be close to the normalized frequency, δ = 1.0 .