The 20 Hz high sampling radar altimetry data and quasi-synchronous satellite images are applied to investigate internal waves in the South China Sea (SCS) and the Sulu Sea. The response characteristics and band sensitivities of radar altimetry data to the modulation of internal waves were analysed. With quasi-synchronous observations, the responses of Sigma0 and significant wave height (SWH) in the Ku band and the C band to the modulation were discussed, respectively. The results demonstrate that for Sigma0, the Ku band is more sensitive, whereas the C band is more sensitive for SWH. The propagation speeds of internal waves in the SCS and the Sulu Sea were calculated with quasi-synchronous observation of radar altimetry high sampling data and MODIS images. The calculated speed of the internal waves is close to the phase speed obtained from the KdV equation. This work indicates that the quantitative parameter of internal waves could be extracted from radar altimetry high sampling data. The altimetry data could serve as an additional data source for the investigation of internal waves. © 2018 Informa UK Limited, trading as Taylor & Francis