We present a quantum interpretation of the heights in hysteresis of Fe 8 molecule at lower temperatures by treating the crystal as an Ising spin system with the dipolar interaction between spins. Then we apply it to two limit cases: rapid and adiabatic regions. Our theoretical analysis is in agreement with the experimental observation in these regions, which indicates that the steps in hysteresis loops of magnetization of Fe 8 at lower temperatures show a pure quantum process.