Aimed at clarifying ��-ferrite formation in fusion welding of ferritic/martensitic steels, effects of cooling rates on ��/�� transformation and martensitic transformation in modified ferritic heat resistant steel are investigated by high-speed quenching dilatometer equipment and microstructure examination. In heating stage, ��-austenite starts to transform into ��-ferrite at 1250 �� 20 ��C. With respect to cooling stage, at cooling rate of 1 ��C/s, �� ��� �� transformation takes place at temperatures above 1250 ��C. Stripe-shaped ��-ferrite is produced at 1 ��C/s, and under higher cooling rates (30, 50 and 200 ��C/s), ��-ferrite exhibits block morphology. The different levels to which the �� ��� �� transformation proceeds would result in the variation in austenite grain size and the amount of ��-ferrite, affecting the subsequent martensitic formation. Effects of cooling rates on martensite start temperature have been discussed in terms of austenite grain size and austenite chemical stability.