Light olefins are important products of high temperature Fischer-Tropsch synthesis over iron catalysts. In this paper, we found that the catalytic performance of iron catalysts was strongly affected by iron nanoparticle size, their encapsulation inside of carbon nanotubes and promotion with bismuth or lead. The presence of promoters and iron nanoparticle confinement leads to a major increase in the reaction rate. A gradual increase in the TOF numbers with the increase in the iron nanoparticle sizes from 2.5 to 12 nm was observed at both 1 and 10 bar over the carbon nanotubes containing encapsulated monometallic or Bi- or Pb-promoted iron nanoparticles. The size of monometallic iron nanoparticles encapsulated in carbon nanotubes does not show any noticeable effect on the light olefin selectivity, while in the Bi- and Pb-promoted catalysts, the light olefin selectivity was higher over smaller encapsulated iron nanoparticles and decreased with the increase in the nanoparticle size. The stability of iron nanoparticles versus sintering was also improved by the encapsulation.