Geostationary Ocean Color Imager(GOCI) is one of three payloads on board the Communication, Ocean, and Meteorological Satellite(COMS) launched 27th, June, 2010. For understanding GOCI imaging performance, 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. We then combined them in Monte Carlo based ray tracing computation. 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 40km in spatial resolution. It is then fed into the instrument before reaching to the detector plane. Trial simulation runs for the GOCI imaging performance were focused on the combined slot images and MTF. First, we used modified pointing mirror mechanism to acquire the slot images, and then mosaiced them. Their image performance from the GOCI measurement were compared to the ray tracing simulation results. Second, we investigated GOCI in-orbit MTF performance with the slanted knife edge method applied to an East coastline image of the Korea peninsula covering from 38.04N, 128.40E to 38.01N, 128.43E. The ray tracing simulation results showed 0.34 in MTF mean for near IR band image while the GOCI image obtained 9th Sep, 2010 and 15th Sep, 2010, were used to produce 0.34 at Nyquist frequency in MTF. This study results prove that the GOCI image performance is well within the target performance requirement, and that the IRT end-to-end simulation technique introduced here can be applicable for high accuracy simulation of in-orbit performances of GOCI and of other earth observing satellite instruments.