This paper focuses on the two domain decomposition methods, the subdomain decomposition iterative method(SDIM) and the characteristics basis function method (CBFM) combined with adaptive cross approximation(ACA) to compute the normalized radar cross section (NRCS) from a perfectly conducting two-dimensional(2D) randomly rough surface. The 3D electromagnetic problem is solved from the electric field integral equationdiscretized by theGalerkin method of moments with the Rao–Wilton–Glisson basis functions. In addition, a parametricstudy versus the number of blocks, the number of overlapping edges, the thresholds of recompressed ACA(RACA; ACAcombined with twoQRdecompositions and truncated by aSVDprocedure, also named ACA-SVD orACA-TSVD) and the parameters inherent to the CBFM is investigated. The complexity of the two methods is alsoaddressed.