The effect of nonmetallic inclusions and micro-structures on the mechanical properties and Hydrogen Induced Stress Corrosion Cracking (HISCC) of welded line-pipe steels have been investigated. Specimens were taken from the welded region of API Grade X-65 and J-55 fabricated by High Frequency Resistance Welding process. Mechanical properties have been examined by means of the measurement of Charpy V-notch impact energy and flattening height. HISCC tests were conducted in $H_2S$ saturated aqueous solution containg 5% NaCl and 0.5\% $CH_3COOH$ at the temperature of 35±2℃ using smooth U-bend specimens. In the present work, incubation and propagation periods for HISC Cracks have been differentiated by means of electric potential method. Calibration of crack length with potential difference was carried out with the least square curve fitting method introducing the new parameter. Ca-treated specimens exhibited not only the longer incubation period for HISC Cracks but also the higher resistance to crack propagation. This is due to the formation of calcium sulfides instead of easily deformable manganese sulfide. The mechanical properties, however, were slightly improved. In the case of pearlite reduced steels, the propagation rate was a little reduced, while the incubation period was prolonged, possibly due to the reduced portion of pearlite. This microstructural feature increased the Charpy V-notch impact energy and markedly decreased the flattening height. In the weldment of line-pipe steels, HISCC kinetics could be divided into the four distinct stages, namely incubation stage, initiation stage, unstable propagation stage, and stable propagation stage. In all specimens, HISC Cracks propagated with transgranular brittle cleavage fracture mode.