Factors controlling critical COD value (δ _C ) of multi-pass submerged arc welded joints of 500 MPa class high-tensile strength steels were investigated through metallographic exa mination of fatigue crack-tip location and fracture initiation points in the COD test specimens. Main results obtained are as follows : Brittle fractures with low δ _C were initiated mostly from the grain-coarsened zone that was reheated between A _{C 1}. and A _{C 3}transformation temperatures and secondarily from the grain-coarsened zone that was reheated below A _{C 1} by the subsequent welding passes. Occurrence of the fatigue crack-tip to the grain-coarsened zone was a prerequisite for low δ _C . Critical COD value was not related with the sum of the lengths of the grain-coarsened zones crossing the fatigue crack-tip front but decreased monotonically with increasing size of the grain-coarsened zone from which fracture was initiated. This result was explained by the following reasons : (1) Larger size of the grain-coarsened zone causes lower reheating temperature of the subsequent welding pass, which results in lower toughness of this zone. (2) Higher tensile stress at the crack-tip is achieved at larger size of the grain-coarsened zone because the grain-coarsened zone has higher hardness than the surrounding regions. Higher crack-tip tensile stress promotes brittle fracture initiation at smaller crack-tip defor mation.