Abstract: In the present study, it was demonstrated that large numbers of mast cells occurred in the duck lymphatic organs. The results obtained with transmission electron microscopy(TEM) and five sorts of histochemical staining methods showed that these mast cells exhibited heterogeneity in shape, size, metachromasia,mucopolysaccharide content of the cytoplasmic granules,staining property and ultrastructure . Histochemical staining revealed that neutral mucopolysaccharide type and acidic mucopolysaccharide type mast cells were present in these organs, and the latter could be classified forther into sulfate mucopolysaccharide type and carboxyl mucopolysaccharide type. In addition, their granules contain bioamines, but probably not heparin. They were varied in shapes and sizes in the thymus,lymphatic node and cloacal bursa, but were smaller , round or ellipse in spleen. Under TEM, these mast cells had a round or elliptic nucleus , and were nearly devoid of processes on the surface, but had a few organelles and numerous scretory granules in the cytoplasm;the granules contain particles,but no scroll or lamellar structures.