In this study, large numbers of mast cells were found in the thymus of goose under transmission electron microscope. They were located mainly in the medulla of the thymus, were round or ellipsoid in shape, had a few cytoplasmic processes, and possessed a rounded, elliptic, or crescent shaped nucleus with distinct nucleoli. Their cytoplasm contained abundant organelles and numerous membrane bound secretory granules that were mostly round in shape and varied in size. According to the ultrastructural features, these granules were classified into 4 types: type 1 granules contained electron dense fine granular material; type 2 granules contained electron dense coarse granular material; type 3 granules contained electron dense concentric beaded chains; type 4 granules showed almost homogeneous high electron density. No crystalline, scrolled or lamellar structures were evident in the granules.