The Mio-Pliocene Miura Group distributed in the Miuraand Boso Peninsula is composed of marine sediments such as silt, sands, gravels and tuff. The tuff beds are the most useful pyroclastic key beds for correlation. Most of pyroclastic key beds are correlated with the stratigraphical combination of the beds which are distinguished each other by the various characters. We investigated the typical pyroclastic key beds (So tuff, Ok tuff, Hk tuff) of the Miura Group, in terms of the following characters : thickness, color, intralayer lithological succession, size distribution, mineral composition, heavy mineral composition, shape of glass and chemical composition of amphibole. The results show that the each bed of the Ok tuff is distinguished by the heavy mineral composition and the stratigraphical combination of each bed is useful for correlation of the Ok tuff. The Hk tuff and So tuff are characterized by the heavy mineral compositions which are abundant in amphibole, The characters of these pyroclastic key beds in the Miura Peninsula are the same as those in the Boso Peninsula. Thus the heavy mineral compositions of the pyroclastic key beds are useful for correlations in the extensive area.