Seven petrified trees (with lengths between 22 and 69 meters and diameters ~2 meters) in the Petrified Forest Park, Tak Province in Northern Thailand were investigated in terms of permineralized minerals via thirty samples by collecting three to five samples from each tree. The trees fell down and were buried in the quaternary palaeoriver gravel beds at the depth less than ten meters. Later, the overburden was removed and now they are exposed to an open atmosphere. Unfortunately, the wood has started degrading due to the extreme weather condition of the tropical monsoon climate of the region. In the petrographic study, each selected sample was cut in three directions (cross section, tangential section, and radial section) related to the trunk, then polished to identify the minerals under a polarizing microscope. Some of the samples were analyzed by scanning electron microscope, X-ray diffraction, and Raman microscope to confirm the results and to provide mineral crystallographic features. It was found that all of the samples contained a quartz mineral in a wood texture. There were large or coarse quartz grains embedded in the vessels compared to other wood cells (e.g. fiber, parenchyma, and ray). Most samples were weathered and there were Fe-oxide compounds in red or reddish brown stains in other cells outside the vessels. In some samples, there were granular quartz grains occupying in the vessels. However, with the grain size of the quartz in the vessels was smaller than the bigger prismatic quartz grains diverging outside the vessel by using the wall of the vessel as a substrate. Some samples have the contortion cells, suggesting that the wood was subjected to a pressure after the deposition. In the cracks or fractures, there were tiny quartz grains filling in and showing the form of drusy aggregate on the wall of the cracks. In certain samples, there was a spherulitic texture of fibrous quartz grains radiating from the vessel, a typical characteristic of the chalcedony quartz variety. The XRD and Raman results confirmed the petrographic results and provided additional information on the weathered part of the wood, also containing clay minerals which were the alteration products of quartz. The result of this study will be conveyed to the conservation research team.