A modified Monte Carlo Metropolis method is performed to simulate the low-temperature magnetization behavior of the ferromagnetic nanoparticles with different sizes and surface anisotropies, based on three-dimensional classical Heisenberg model. It is found that an "insulating" layer appears to cut off the interaction between the surface and the core as the surface anisotropy is strong enough. The results also indicate that the magnetizing state of nanoparticles may change from a "collinear" structure into a "throttled," then into a "hedgehog-like," and finally into a "separation" structure with the increase of surface anisotropy. However, the "hedgehog-like" structure does not appear in the bigger size nanoparticles. The energy competition between the surface and the core in the ferromagnetic nanoparticles may be the reason for producing these phenomena.