A nonlinear detection structure is systematically developed for sinusoid in additive unknown colored Gaussian noise. The proposed scheme has made use of the difference of the spectral peakedness of the observation date. Nonlinear spectrum estimator is employed to evaluate the peakedness of the spectrum. Simulation results indicate that, in addition to some attractive robust features, the proposed scheme also achieves considerably better performance than the conventional energy detector in terms of the error probabilities.