  We study effects of an external magnetic field on charge ordering in the
one-dimensional extended Hubbard model at quarter filling by means of the
quantum Monte Carlo method. We find that the Zeeman coupling enhances the
charge order correlation, which is prominent when the system is located in the
critical regime near the charge ordering transition at zero magnetic field.
This behavior is interpreted in terms of the crossover to the fully
spin-polarized limit where the model is exactly solvable. Furthermore, by
incorporating the interchain Coulomb repulsion, we show that the
charge-ordering transition temperature is raised by the magnetic field. We also
discuss the relevance of our results to magnetoresistance effects observed in
molecular conductors.
