A physical picture of spin alignments in organic molecule-based ferrimagnets is deduced from quantum Monte Carlo (QMC) simulations of magnetic susceptibility. It is found from the QMC calculations on the basis of a Heisenberg spin Hamiltonian that the ferrimagnetic spin alignment in S=1 and S=1/2 alternating molecular chains is equivalent to a ferromagnetic alignment of effective S=1/2 spins appearing in the unit cells of the heteromolecular chains. A spin polarization effect affording the effective ferromagnetic interactions is demonstrated in terms of a Heitler-London approximation.