Abstract: In order to study and compare the structural characteristics of Tridacna and its similar shells such as pearl oyster, Cypraecassis rufa and Peristernia incarnate, and further more to explore the influence of the structures on the appearance,we reaserched these four kinds of shell samples by using polarizing microscope and scanning electronmicroscope.Though these shells all show layered structure on the macro, the microstructure is obviously different.There are two kinds of layer structure in the Tridacna shell:Prismatic structure & cross structure. This is the reason why Tridacna have different stripes in different transparency and colours.While the pearl oyster have a typical brick structure, thus it shows pearly luster.The transparency of pearly oyster depends on the thickness of the slate tablet.Cypraecassis rufa and Peristernia incarnate show arc-parallel layered structure under unaided eyes.There are two sets of interactive layers in each layer consisting of two perpendicular prismatic layers. This special structure results in the "falming" structure in some particular direction of these two shells.