Effects of potassium or lanthanum additives on the catalytic properties of supported cobalt catalysts were examined through CO hydrogenation reaction. The catalysts were characterized by $H_2$ or CO chemisorption, $O_2$ titration, TEM and TPD. The reactions were carried out at 270℃ under 1 atm as well as 10 atm. By adding small amount of potassium to alumina supported cobalt catalyst, $H_2$ adsorption was reduced more significantly than CO adsorption, and the extent of reduction to cobalt metal was also reduced. With potassium added, the overall CO conversion decreased, while the selectivity for higher hydrocarbon and olefin increased. Addition of lanthanum showed a similar effect on catalytic properties, but its effect was less significant than that of potassium. The temperature-programmed desorption showed that the adsorbed state of CO on cobalt were changed due to the presence of additives.