Laser-arc　hybrid　welding　can　improve　production　efficiency,　and　thus,　it　is　being　used　in　an　increasing　number　of　industries.　Herein,　the　effects　of　laser-arc　hybrid　welding　process　param-eters　on　the　microstructure　and　mechanical　properties　of　welded　joint　of　low-and　medium-carbon　steels　were　investigated.　Welding　seam　with　best　quality　and　no　defects　was　obtained　at　welding　current,　arc　voltage,　laser　power,　and　welding　speed　of　260　A,　24.8　V,　3300　W,　and　0.8　m/min,　respectively.　Moreover,　the　enrichment　of　Mn　in　the　weld　zone　promoted　bainite　formation.　Rapid　cooling　led　to　the　pseudo-eutectic　phenomenon　in　the　heat-affected　zone　(HAZ)　on　both　sides　of　the　weld;　therefore,　when　an　HAZ　was　closer　to　the　weld　on　both　sides,　the　pearlite　content　was　higher.　For　low-carbon　steel,　the　microstructure　of　the　HAZ　comprised　ferrite,　pearlite,　and　ferrite　widmanstatten.　For　medium-carbon　steel,　the　microstructure　of　the　HAZ　comprised　a　large　amount　of　pearlite　and　a　small　amount　of　ferrite.　In　welded　joint　of　both　types　of　steels,　differences　in　microstructure　and　internal　stress　altered　the　nanostructure　morphology　and　distribution　characteristics　of　cementite.　The　welded　joint　also　had　considerably　better　tensile　strength　due　to　a　large　number　of　dislocations　and　better　grain　refinement,　which　led　to　tensile　fractures　in　the　base　metal　zone　of　low-carbon　steel.　For　a　prefabricated　weld　notch,　the　tensile　fracture　mode　was　a　ductile　fracture　with　massive　dimples.