The characterization of near-infrared (NIR) mesoporous silica nanoparticles (MSN) suitable for in vivo optical imaging with high efficiency is presented. Trimethylammonium groups modified MSN (MSN-TA) with the average size of 50-100 nm was synthesized with incorporation of the TA groups into the framework of MSN. It was further adsorbed with indocyanine green (ICG) by electrostatic attraction to render MSN-TA-ICG as an efficient NIR contrast agent for in vivo optical imaging. The studies in stability of MSN-TA-ICG against pH Indicated the bonding is stable over the range from acidic to physiological pH. The in vivo biodistribution of MSN-TA-ICG in anesthetized rat demonstrated a rather strong and stable fluorescence of MSN-TA-ICG that prominent in the organ of liver. Transmission electron microscopy (TEM) imaging and elemental analysis of silicon further manifested the physical and quantitative residences of MSN-TA-ICG in major organs. This is the first report of MSN functionalized with NIR-ICG capable of optical imaging in vivo.