A novel design approach to wideband, dual-resonant microstrip annular sector patch antenna (MASPA) is proposed. As mathematically revealed by the dominant eigen-equation, MASPA with increased backfire radiation can be evolved from a truncated microstrip leaky-wave antenna. Then, the general relationships between the ratios of outer/inner radii, flared angles, and operational modes are deduced. The outer arc of the annular sector patch is set as a magnetic wall or open-circuited while the inner one is set as an electric wall or short-circuited, such that the patch’s size could be accordingly reduced by 13% to the circular sector case. As finally validated, a dual-resonant MASPA with the flared angle of 150°, outer/inner radii ratio of 3.0, and backfire gain up to 4.0 dBi can be implemented on single-layered air substrate with a height of 0.043-wavelength at the center frequency. Therefore, the advanced design approach can effectively realize patch miniaturization, wide band and increased backfire radiation without inquiring extra multiple radiators and incorporating any additional reactance compensation techniques.