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A two-dimensional nonuniform fast Fourier transform (2-D NUFFT) technique is developed for analysis of microstrip circuits in a rectangular enclosure. The 2-D Fourier transform of a nongrid point is approximated by Fourier bases in a square neighborhood with (q+1) by (q+1) grid points. The square neighborhood can be reduced to an octagonal region with q2/2+3q+1 grid points without sacrificing accuracy if q is sufficiently large. This technique allows an arbitrary discretization scheme on conductors and shows a great flexibility for the analysis. Asymmetric rooftop functions are inevitably used to expand surface current densities on conductors. Based on the spectral-domain approach, all elements of the final method-of-moments matrix are double summations of products of a weighted Green's function and trigonometric functions. By using the proposed technique, the double summations at all sampled points can be obtained via the 2-D NUFFT. The scattering parameters of a compact miniaturized hairpin resonator, an interdigital capacitor, and a wide-band filter are calculated. The calculated results show good agreement with measurements.