A new method that optimizes a control of hydrocarbon (HC) addition to diesel exhaust gas for HC type DeNOx catalyst system has been developed. These catalysts are called as the HC-DeNOx catalyst in this paper. The system using HC-DeNOx catalyst requires a resonable quantity of hydrocarbons addition in the inlet gas of the catalyst, because the HC concentration in a diesel engine is so low that the HC is not sufficient for NOx conversion. Generally ambient temperature in the exhaust manifold is $250{sim}350^{circ}C$, so spray behavior in this case is different from that of any other condions. This research shows spray behavior of injected hydrocarbons in the transparent exhaust manifold.