Thick sequences of Karaj green tuffs with the age of Eocene are dominant in north of Tafresh, Central Iran. These pyroclastics consist mainly of lithic tuff, crystal tuff and vitric tuff. XRD, SEM and EMP analysis as well as thin section petrography indicate that a widespread alteration has resulted in the formation of a variety of low grade metamorphic and secondary minerals such as sericite, calcite, chlorite, albite, analcime, epidote, prehnite and pumpellyite. This mineralogical assemblage suggests that the tuffs, in their basal horizons, have undergone a low grade metamorphism in the zeolite and prehnite- pumpellyite facies. It is likely that the considerable thickness of tuff series is responsible for such a burial metamorphism in the north Tafresh area.