The chemical process of CN, formation in a CO-N-2 mixture is studied through temporally resolved intensity measurement of CN violet radiation occurring in the reflected-shock region of a shock tube. A 78% CO-22% N-2 mixture is driven by cold hydrogen to a shock speed of up to 3.45 km/s, to produce-a reflected-shock temperature corresponding to Martian entry flight of up to 6.4 km/s. Absolute calibration of spectrometer is performed using a standard lamp of radiance. A reaction model is constructed by combining four existing models and multiplying the C-2 dissociation rate by a factor of five.