In this study the effect of α-FeSi2 and ε-FeSi nanowires diameter on the lattice thermal conductivity was considered. Ab initio modeling was performed in the temperature range of 100–700 °K and nanowires diameter range of 6–48  nm. Results showed that at minimal considered diameter nanowires have 1.4–4.5 times lower lattice thermal conductivity than bulk material depending on temperature and nanowire elongate direction.