Abstract In this work we address the effect of surface anisotropy upon the magnetic structure of ferrimagnetic maghemite γ-Fe 2 O 3 nanoparticles in the limit of low temperatures. Our model is based on a three-dimensional classical Heisenberg–Hamiltonian involving Fe 3+ nearest-neighbor interactions, surface and core anisotropies, and a Monte Carlo-Metropolis approach via simulated annealing for energy minimization. Results reveal a marked decrease of the Curie temperature of the considered nanoparticle with that obtained of a bulk maghemite, as well as severe variations with respect to a single domain phenomenology as surface anisotropy increases.