For utilizing optogenetics in neuroscience research, a proper setup is necessary, which delivers sufficient light to target cells and minimizes unexpected side effects caused by light exposure. In this study, we were interested in the area of influence of optical surface stimulation on a spinal cord tissue. We built a 3D spinal cord structure of rat and utilized the Monte-Carlo methods to simulate the light transport in it. We first evaluated light propagation in homogeneous nervous tissue models. For a 10-mW, 470-nm light source, light intensity of 1 mW=mm2 was detected at depths of 1:14 and 1:77 mm in white and grey matter, respectively. This indicated a narrower spreading pattern of light in the white matter than in the grey matter. Since the grey matter, which contains the somatosensory pathways, is an important target of spinal cord stimulation, we focused on investigating how much light could reach this area in a multi-layered structure. The results showed that when an optical fiber was positioned in the center line of the spinal cord dorsal surface, most of the light energy was absorbed before reaching the grey matter. In contrast, when we put the fiber on a lateral position, 0:8mm away from the central line, relatively sufficient light intensity could be detected deep into the lamina 5 area. The experimental results obtained herein suggest that tissue type and the position of stimulation could greatly affect the area of influence of light stimulation in a 3D spinal cord. It is important to consider the location of the interested neural pathways and plan a proper stimulation site before conducting optogenetic surface stimulation of the spinal cord.