Effects of aging treatment on high temperature strength of Nb added ferritic stainless steels for automotive parts were investigated. Hot tensile tests were carried out at 700 degrees C after the aging at 700 degrees C for different aging times using Gleeble 1500. High temperature strength of all steels decreased as the aging time increased. In Nb free steels, the reduction in high temperature strength is mainly due to grain growth. On the other hand, in Nb added steels, the reduction in high temperature strength occurred by Nb precipitation. It was observed that Fe2Nb (Laves phase), Nb(C,N) and Fe3Nb3C were precipitated out during the aging at 700 degrees C in Nb added steels. The coarsening rate of Fe2Nb was higher than that of Nb(C,N). Fine Fe2Nb precipitates formed during at the early stage of aging contributed to high temperature strength in 0.01C-0.38Nb steel. However, coarse Fe2Nb particles formed during the aging were very detrimental to high temperature strength. The coarsening of Fe2Nb was relatively retarded by adding Mo.