Several primary dolomite samples of the Upper Sinian T)en}yin} Formation and replacement dolomite samples of Ordovician were selected in Yan}tie area for analysis. The high resolution TEM (Transmission Electron Microscope) was applied to make a microstructural correlalion between primary dolomite and replacement dolomite crystals. It has been revealed that microstructures (mainly dislocation and modulated structure)are quite common in fibrious and bladed dolomite cements, and crystal structures are heterogeneous. On the contrary, microstructures are relatively simple in micritic dolomite and coarse-gained dolomite cement and crystal structures are comparatively homogeneous. The main dislocation in primary dolomite crystals is screw dislocation. The interaction between dislocations results in the formation of dislocation wall or network which is relevant to extinction direction. The screw dislocation is formed by crystal face sliding during crystal growth at low-medium temperature rather than resulting from later deformation. In diffraction contrast, the modulated structure in primary dolomite appears locally and exhibits a somewhat regular alternating lamellae. The wavelength (modulated period width)is 10-30nm wide.The modulation is oriented with the long dimension always parallel to the growth normal.SAED (Selected Area Electron Diffraction) pattern exhibits ultra reflection ( Type c and Type d).The modulated structure in primery dolomite is of density-modulation (composition-modu-lation)，which is formed during crystal growth instrad of by later solid-state diffusion. The main growth of primary dolomite crystal is in orientation to (1014)，and the modulation is perpendicular to (1014). Microstructures are relatively rare in replacement dolomite crystals and mainly distributed in Ca-rich salt dolomite in which the modulated structure is perdominant. The main dislocaLion is blade dislocation which is formed as a result of later deformation. The modulation in replacement dolomite differs from that in primary dolomite in that it exhibits irregular lamellae and vermiform with poor continuity. Sometimes two modulated lamellae intersect each other.The wavelength varies considerably. The modulation is pervasive throughout a crystal. It is formed during the replacement of calcite by dolomite relevant to the corporation of excess Ca in Mg sites. According to EDS analyses , in primary dolomite, the Ca content in modulated area is slightly higher than that in unmodulated area. Mg contents in both areas are identical.In replacement dolomite Mg and Ca contents in modulated area are similar to those in primary dolomite, but the unmodulated area is almost full of Ca with merely a little Mg. Relict calcitecoexists with modulated dolomite. TEM has revealed that the microstructures in primary dolomite and replacement dolomite are different suggesting their different geneses.