We have developed retrieval methods for swell parameters such as wave height, wavelength, and wave propagation direction from images of ALOS/PALSAR taken over coastal seas around Japan. The retrieval methods for wavelength and wave direction used the PALSAR image spectrum. Using the coupling swell spectra and the PALSAR image spectra, an empirical L-band modulation transfer function (MTFL-band) was obtained for the wave height retrieval through linear regression analysis. A comparison between SAR-derived and in situ wave parameters showed close agreement. For the wave direction, the bias, root mean-square error (RMSE) and correlation coefficient were 1.3°, 15.5° and 0.94, respectively. For the wavelength, the bias, RMSE and correlation coefficient were 10.4 m, 18.3 m and 0.93, respectively. These results demonstrate that the L-band SAR imaging mechanism for swells is mostly linear. For the wave height, the bias, RMSE and correlation coefficient were 0.08 m, 0.30 m and 0.80, respectively.