Abstract: A close relationship between a ship's length and its wakes is presented, and a model is built up based on it. Under the thin ship assumption, the feasibility of estimating ship's length from its wave spectra is discussed mathematically, then a three steps of method estimation is introduced. This method is compared with the traditional method and the ellipse method. The results show that the error of the new method is less than 5%. It is less affected by orientation and is more robust than the traditional methods, and it helps to recognize or classify ship targets in SAR images in the future.