We observed sea clutter using an L-band long-range air-route surveillance radar (ARSR) having a frequency 1.3 GHz, a beamwidth 1.2°, and a pulsewidth 3.0 μs. To determine the sea clutter amplitude, we introduce the Akaike Information Criterion (AIC), which is more rigorous fit of the distribution to the data than the least squares method. It is discovered that the sea clutter amplitudes obey the log-normal, Weibull, and log-Weibull distributions with the shape parameters of 0.668 to 0.989, 5.18 to 8.30, and 1.07 to 7.82, respectively, for data within the beam width of an antenna.