Abstract: Formastor-D autodiatomtry was used to determine the continuous cooling transformation diagram of three types of 9%Cr-1%Mo weld metals.Optical microscopy and Transmission electron microscopy were employed to observe the characters of the microsturcture transformation at different cooling rates.The results showed that the austenite of all three weld metals transformed to martensite at a very wide range of cooling rate.It could transform to pre-eutectoid ferrite only at a quite slow cooling rate.Among those weld metals,the austenite of No.2 weld metal had the fastest cooling rate for pre-eutectoid transformation.It was found that the martensite in three weld metals was lath martensite with high density of dislocations.Auto-tempered cementite (Fe₃C) and residual austenite could also be found in martensite.The pre-eutectoid ferrite transformation initiated at the original austenite grain boundaries and formed a net structure.In pearlite,the carbide was coarse spherical M₂₃C₆.