For garnet amphibolites from the same area, Panjasawatwong (1991)reported the chemistry of coexisting·metamorphic minerals and calculated the equilibration temperature to be 600-780 ·c at 5 kbar. The crossite-epidote blueschists, on the other hand, were inferred from a petrogenetic grid to have formed at a pressure of about 7 kbar and a temperature in the range 390-450 ·c. In many regions around the world, metamorphic study has been proven tobe very useful to complement structural work and aid interpretation of relevant tectonic processes (e.g. Turner, 1981; Berry and Grady, 1981; Kemp et al., 1985; Jaygo and Blake, 1989; Gottschalk, 1990). Understanding the metamorphic history of the Sirikit Dam area is an important additional constraint in the reconstruction of tectonic scenario of the northern Thailand region. The aims of this chapter are to document the P-T conditions and timing of metamorphism of the important metamorphic units and to outline the metamorphic history of the Sirikit Dam area.