Weld joint performances of X80 line pipe welded using full-automatic Gas Metal Arc Welding (GMAW) and manual Shielded Metal Arc Welding (SMAW) were evaluated. To clarify effects of welding conditions on mechanical properties and microstructure of weld metal, mechanical tests and metallurgical surveys were conducted. For GMAW, strength of the weld metal decreased with increasing heat input. This was due to the increase of acicular ferrite lath width and percentage of grain boundary ferrite and ferrite side plate. Good Charpy impact toughness was obtained for all the welding conditions, which was caused by the high proportion of fine acicular ferrite. For SMAW, the strength and Charpy impact energy of the weld metal decreased as the heat input increased. Overmatching for the specified minimum yield stress (SMYS) was obtained for all the welding conditions for both GMAW and SMAW. If distribution of the strength of X80 pipes was taken into consideration, however, overmatching was obtained only in the lower heat input range.