Magnetic ac susceptibility of randomly arranged crystallites (powder sample) and well-oriented Fe8 crystallites has been measured as a function of applied magnetic field, temperature, and frequency. The Fe8 clusters, made of eight iron ions and with formula [(tacn)6Fe8O2(OH)12]8+, have a ground state S=10. From the magnetization data M(H) with the dc field, H, applied parallel and perpendicular to the easy axis of the aligned sample and χ″(T) data for different frequencies, the anisotropy field Han∼5 T and energy barrier U/kB=26.9 K were obtained. Peaks in the curves of χ′(H) were clearly observed at fields Hn=nH0, with n=0,±1 for powder sample where H0=2.4 kOe; and n=0, ±1, ±2, for an oriented sample where H0=2.2 kOe. These peaks appearing at Hn=nH0 in the curves of χ′(H) can be well described by the thermally assisted resonant quantum tunneling of magnetization.