A series of bifunctional 20Fe-Cu(x)-4K/ZSM5 (x = 0, 2, 4 and 6 wt.%) catalysts were prepared to elucidate the effect of Cu promoter on F-T synthesis functionality. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature-programmed reduction, thermal analysis and temperatureprogrammed desorption of NH3. Although addition of Cu essentially increases the initial activity, its effect is not significant at steady-state reaction. At 2 wt.%Cu loading, the homogeneous mixing of Fe and Cu oxides results in iron carbide formation, however, beyond this loading phase segregation occurs and eventually decreases the reducibility and acid site density, as reflected in the activity and selectivity.