Nonparametric spectral estimators have many applications including target range signature estimation and synthetic aperture radar (SAR) imaging. In this paper we explain how to apply nonparametric approaches to SAR imaging. We present an important nonparametric spectral estimation method which is Capon algorithm. We compare the Capon method with other nonparametric methods such as APES, Periodogram, and FFT for their resolution, sidelobe levels, and spectral peaks. And we also show by means of experimental examples that the forward-backward Capon gives much better estimation than the forward only Capon.