Abstract: The investigations in the past show that the weld metals of 15MnVN steel by SAW process with high heat input usually have over high strength and comparatively low toughness. In this paper, the precipitatien eehavior of V and the existing forms of V and N in the weld metals of Mn-V-N and Mn-Mo-V-N systems were investigated, with STEM and EDAX methods. Their effects on the strength and the toughnes of the weld metals were studied. This werk proved that under as-welded or SR treatment conditions V and N in the weld metals do not precipitate incoherently since the cooling rate of the weld is so high and the reheat temperature is so low. The coherent precipitation of V is the main factor which makes the steength of the weld metal very high but the toughness comparatively low.