Abstract: Some micro-structures of Melosira varians, M. jurgensi, M. undulata and M. undulata var. normanni recovered from Miocene to Quaternary deposits in China are observed under SEM. They show that: 1). The frustule wall of M. varians is composed of two thin layers, the inner wall and the outer wall. The outer wall has two kinds of pores: micropores with a diameter of 15–60 μm, widely distributed, and mega-pores of irregular shape with a diameter of 300–800 μm, sparsely distributed on valve and girdle surfaces. The inner wall has only micro-pores with a diameter of 100 mμ. Linking spines are not observed on the valve edges of these specimens. 2). Obliquely directed lines occur on girdle surfaces of M. jurgensi. They are arranged more regularly near sulcus, and discontinuously distributed near valve surface. There are very sparsely distributed micro-pores on valve surface, and linking spines present and “T” shaped. Auxospc,res are long cylindrically shaped. 3). Two distinctly different structures are observed on girdle surface of M. undulata under SEM. One of them is lines arranged parallel to pervalval axis under light microscope. The other is smooth in appearance under low magnification, and lines composed of minute micro- pores may be observed. When magnified over × 5,000, there are about 90 lines within 10 μ. Besides, old valve jacket may be still preserved outside valves of these specimens. The upper parts of old valve jackets are composed of comparatively wider long- tudinal lines, while the lower parts composed of crossed lines (lateral and longitudinal) of same width. Linking spines are shaped like dogs teeth. There are also two layers (inner and outer) in valve walls of M. undulata var. normanni. The outer layer is composed of thinner wall of same thickened, while the inner layer of thicker well of irregular thickenes. A hillshaped protrusion may be observed at joints of various thickening wavy line as shown by the valve's dorsal view.