The low‐temperature experimental study of the antiferromagnet FeCl2in pulsed fields up to nearly 500 kOe has given evidence of a nonlinear behavior of the magnetization when the field is applied perpendicular to the anisotropy axis, the magnetization versus field increasing more steeply near the saturation than one would expect from the usual molecular field theory alone. A simple microscopic explanation of this phenomenon (which has already been observed in many antiferromagnets) can be put forward in this particular case: the essential in FeCl2lies in the existence of comparable orders of magnitude for crystal anisotropy and exchange interactions. At 0°K, by using a crystal‐field approximation (strong cubic potential and weak trigonal distortion), and a molecular‐field approximation (Heisenberg Hamiltonian with two exchange parameters |JF|≫|JAF|), we get a nonlinear magnetization law before the saturation. The calculation has been carried out thoroughly only in the case where the ferromagnetic exchange is large compared with the crystal anisotropy. The value of the exchange and crystal‐field parameters of FeCl2lead to a satisfactory quantitative agreement between experiment and theory.