In order to understand the mechanism of the bending motion of the electroactive polymer actuators from the molecular interaction, we performed Monte Carlo simulations in two length scales; the micrometer scale and the nanometer scale. In the micrometer scale picture, the bending motion of an actuator can be viewed as the inhomogeneous expansion/contraction of the three layer system. We theoretically formulated the deformation of the actuator in terms of the elastic constants and the stress exerted due to the applied voltage. For the nanometer scale, noting that the electrodes of the EAP actuators have porous structures, we modeled the anode and the cathode by the porous electrodes where the ions are confined in the space with the dimension comparable to the ion size. We found that significant osmotic stress arises in the porous electrodes when voltage is applied. The results of such multi-scale analyses are combined with the experimental results to obtain the insights into the molecular mechanism of the actuators and to give the guideline for the molecular design of the actuators.