Western metamorphic belt in Thailand elongated in North-South direction has been grouped into the Chiang Mai-Lincang belt. As part of this metamorphic belt, the Lan Sang National Park located in the west of Tak Province is an affected area from northwest-southeast striking Mae Ping fault. Along the fault zone, rock formations have been strongly deformed, yielding steep foliation and lineation. Based on petrographic study, these rocks are mainly characterized by gneisses showing very fine-to coarse-grained textures which consist of quartz, plagioclase, K-feldspar, biotite, pyroxene, and chlorite. Calc-silicate and impure marble are partly interlayered in these gneisses; they are composed of calcite, quartz, feldspar, pyroxene, and garnet. Moreover, granitic gneiss, which cuts into the host gneisses and calc-silicate, appears to have a similar composition of gneiss. Based on geothermobarometry, these rocks should have undertaken high-grade metamorphism of lower amphibolite facies with P-T (pressure-temperature) ranges of 570-590°C and 5.8-8.1 Kbar for gneiss. These rocks appear to have several evolution stages. Sedimentary protoliths of gneisses and calc-silicate may have situated in the area prior to regional metamorphism. Subsequently, dikes and veins crosscut into these gneissic and calc-silicate rocks before later metamorphism appears to have continued and reached the ductile-brittle stage.