The trap effects on the formation of the space-charge field (SCF) associated with photo refractivity of non-linear optical polymers were studied by Monte Carlo simulation. The temporal behavior of SCF on the energy disorder is discussed. The charge transport dynamics influenced by the presence of the trap molecules controls the formation of SCF via two mechanisms, the charge separation and charge recombination. As a result, there exists an optimal condition on a certain doped trap molecule for the efficient formation of SCF. The optimal condition as a function of the trap depth is presented to show a linear dependence. (c) 2005 Published by Elsevier B.V.