Power devices with high-performance require long carrier lifetimes within their silicon crystals. This paper reports the in-situ measurement of carbon monoxide in a Czochralski growth furnace of silicon single crystals. Moreover, this paper reports analytical investigation on contamination to silicon melt as functions of pressure in the furnace, argon gas flow velocity and gap width between the melt and a thermal shield. The experimental results show the carbon contamination to the melt increases when the pressure increases and the flow rate decreases. Increase of the gap width increases the contamination of carbon. We could explain the results qualitatively using a simple transport model.