A method of estimating the acicular ferrite transformation ratio of welded metal portion easily with high precision. There is provided a method of estimating the acicular ferrite transformation ratio of welded metal portion produced through a single path or multiple paths, comprising (a) means for computing specified fundamental information and (b) means for estimating the acicular ferrite transformation ratio of welded metal portion, wherein the means (b) comprises dividing the cooling temperature history of the means (a) by microtimes and computing in sequence the acicular ferrite transformation ratio at temperature T (XAF(T)) for each of the microtimes to thereby attain estimation of the acicular ferrite transformation ratio of welded metal portion at the time of completion of final welding path, and wherein the acicular ferrite (AF) has a morphology expressed by the product of side surface area (A(msup
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