The equilibrium magnetization of grain-aligned HgBa 2 Ca 4 Cu 5 O 12+y ͑Hg1245͒ with T c Ӎ 108 K was measured. The Ginzburg-Landau parameter ͑͒, obtained by using the Hao-Clem model, increased unphysically with increasing temperature for T / T c ജ 0.7, which indicates that the fluctuation is greatly enhanced. The two-dimensional scaling behavior of the magnetization and the temperature variation of the irreversibility field H irr ͑T͒ from a power-law behavior at low fields to an exponential behavior at high fields indicate that Hg1245 is highly anisotropic, which seems to be affected by the appearance of antiferromagnetism at the inner CuO 2 planes while blocking the coupling of the superconductivity between the outer CuO 2 layers.