Image mosaic technique is widely used in a field of remote sensing research. However, in case of Geostationary Ocean Color Imager's (GOCI's) mosaic image which is consist of 16 slot images, the radiance level discrepancy was noticed in the cloudy circumstance next to each other slot when acquiring the imagery data in the low Sun elevation angle. We provided, in this study, the in-depth stray light analysis results in order to find out this discrepancy phenomenon, and performed to compare the stray light pattern via a bright target movement. Stray light analysis as the first step was completed with ray tracing technique based on ASAP program, and we suggested that unwanted radiations from the Earth bright target or the atmosphere such as cloud are major candidates of stray light in the problematic images. For embodying GOCI operational concept, we constructed the Integrated Ray Tracing model consisting of the Sun model as a light source, a target Earth model, and the GOCI optical system model. In the second step, we investigated the stray light pattern at each slot image including unwanted random source from out of field, and then constructed the simulated mosaic bias image reached at the detector plane. In the simulated bias, the ray path followed the procedures that light travels from the Sun and it is then reflected from the Earth section of roughly 2500km* 2500km in size around the Korea peninsula with 16 slots. Lastly, we analyzed stray light pattern in the third step for the real image environment acquired at UTC-03 16th, October, 2011. In addition, verification was performed to compare the difference among slot boundaries for moving bright target.