Abstract: In addition to the original deposition, replacement dolomite is the main origin of dolomite. The study of crystal structure of replacement dolomite with different burial stages can help analyze the development process and main influencing factor of replacement. Three kinds of replacement dolomite were chosen for study – the Miocene Cayman Formation in Cayman Island of the Caribbean Sea, the Miocene Xuande Formation in South China Sea and Sinian Dengying Formation in Sichuan Basin. They were replaced from original calcite and not affected by terrestrial materials, with the original grain structures kept. The X-ray diffraction is used to analyze the mineral contents, ordering degrees and crystal cell parameters and the TEM is used to observe the microtopograph of dolomite crystals. With the burial depths ranging from several meters to hundreds of meters and even thousands of meters, there are two typical changes. One is that the crystal forms change from euhedral-subhedral-anhedral, reflecting the restrain of the burial diagenesis on development of replacement dolomite. The other is that a gradually higher ordering degree with crystal cell parameters closer to ideal values reflects that stable burial diagenesis is propitious to the transformation of dolomite from disorder to order. There is no inevitable relation between crystal form of dolomite and the ordering degree. With the X-ray diffraction used, the high-magnesium calcite, high-calcium dolomite and low-calsium dolomite were tested in Cayman Formation samples. The petrographic characters and crystal structures reflected the replacement dolomite experienced the changes from highmagnesium calcite to high-calcium dolomite until low-calcium dolomite, which can be considered as a gradual process that magnesium ions were involved into the crystal lattice of calcite. After a stable development for a long period of time, these three kinds of replacement dolomite gradually became ideal dolomite. The magnesium ion sources of the three kinds of replacement dolomites were nothing but seawater. The paleokarstification catalyzed the replacement dolomitization to a certain extent so that the original porosity is not easy to be destroyed after dolomitization, forming a good reservoir space for oil and gas migration in the later periods.