In laser welding of steel plates for more than 2mm in thickness, it is necessary to use high-power continuous-wave lasers to maintain the keyhole in the depth of the workpiece. Due to high thickness of the sheets and increase in the input power, defects e.g. underfill, root humping, and porosity in the weld depth will increase, lead to inhomogeneity of the weld, and reduce its mechanical properties. Therefore, determination of appropriate laser parameters is highly effective and plays a crucial role in reducing those defects. In this paper, the effect of laser parameters such as its power, welding speed, beam diameter, and the gap between two plates in butt welding of the 5mm-thick steel plates has been investigated using a 3-kW fiber laser.