Abstract Reversible magnetization with the external magnetic fields of 1 T ≤ H ≤ 5 T parallel to the c-axis has been measured for the grain aligned HgBa 2Ca 3Cu 4O 10+δ. A strong vortex fluctuation effect was clearly observed and the magnetization is well described by the vortex fluctuation model. From this analysis, the penetration depth λ ab(0) = 1583 A ̊ and the effective interlayer spacing s = 44.6 A ̊ were estimated. However, the value of s is significantly larger than the lattice parameter c = 19 A ̊ , which is different from the prediction of the vortex fluctuation model. From the model on superconducting fluctuations proposed by Koshelev, in which not only the critical fluctuations at the lowest Landau level but also the Gaussian fluctuations at higher Landau levels were considered, the different value of s = 15.4 A ̊ was obtained.