The effect of Al content on the microstructure and ductile-brittle transition temperature (DBTT) of 18Cr–Al–Si ferritic heatresistantstainless steel have been studied by Charpy impact testing over temperatures ranging from 20 to 90 ℃. Charpy impacttest results show that DBTT increased with increasing Al content. Meanwhile, fracture morphology changed with increasingAl content, where the amount of dimples decreased and cleavage facets increased, indicating increases in the tendency forbrittle fracture. Furthermore, ferrite grain size, volume fraction and size of (Cr, Fe)23C6 carbides increased with increasingAl content, which led to decreasing toughness and increasing DBTT.