This article is concerned with analytical model for radiation pattern of a new double-flared diagonal horn antenna using ''plane wave spectra technique for 3D fields''. The double-flared diagonal horn antenna is accomplished by adding one more set of flares in Eand H-planes with equal flare angles into conventional diagonal horn. The copolarized and cross-polarized radiation patterns in E-, H-, and D-planes have been computed utilizing the analytical model and reported in the article. The parametric studies on peak cross-polarization level have carried out. The radiation performance of double-flared and conventional diagonal horns for same length and aperture size are also compared. It is investigated that double-flared diagonal horn antenna posses better peak cross-polarization level (%216.5 dB) in D-plane (6458 plane) in comparison of conventional diagonal horn (%215.5 dB) at design frequency and retains almost circularly symmetric radiating beam at lower values of elevation angles. Also, double-flared diagonal horn has better matches with free-space and has slightly lower gain ( 0.5 dB) in comparison of conventional diagonal horn. The work presented here can provide useful design guidelines for development of prototypes of double-flared diagonal horn, which may find potential application in satellite communication and imaging applications etc. V