Low carbon (C about 0.1%)-12%Cr heat resisting steel is a martensitic stainless steel with both high strength and ductility given by hardening and tempering and is widely being employed as steam turbine blade steel.This steel, however, has what is called “impact transition temperature”, that is to say, impact energy rapidly decreases near room temperature and embrittles. And this is one of the demerits of this steel.After investigation of various properties of steel into which Nb is being individually and slightly added (less than 0.15%) to stabililize and refine N in the steel for the main purpose of improvement of this demerit it was recognized that the addition of about 0.02∼0.05% makes the impact transition temperature lowered as much as about 40°C and improves the ductility remarkably and at the same time the addition raises strength and creep rupture properties can be improved.Based on this investigation it was able to obtain stabilized quality in manufacturing 0.1%C-12%Cr steel.