A sparse-aperture design named dual three sub-apertures, which is composed of nine sub-apertures, is proposed. The pupil function, analytical formulas of its corresponding modulation transfer function (MTF) and three configuration models have been given. The comparisons among the MTFs of three models are made. The features of three kinds of configuration of dual three sub-apertures is illuminated. The maximum cut-off frequency and equivalent diameters under the different filling factors of dual three sub-apertures for model Ⅰ are given by calculating. The image quality is evaluated by two kinds of method. The image simulations, noisy image restorations and reconstructuring image show that the image spatial resolution of dual three sub-apertures approaches the resolution of encircled aperture, the image quality degrades with the decreasing of filling factor. The image quality can be improved using Wiener filtering. The sparse-aperture system of dual three sub-apertures has the advantages of flexible configuration and easy fabrication.