In this study, fundamental experiment was conducted with various strength of UHSS (Ultra High Strength Steel) by $CO_2$ laser. And then, butt and lap joint laser welding with boron alloyed steel and Al-Si coated boron alloy steel have been done by changing laser beam feature, existence of gap and existence of coating layer to know welding characteristics of those materials. As a result, in case of fundamental experiment with various strength steel, hardening was found in the weld metal of all tested materials and softening was found at the heat affected zone of SGAFC 1180. In case of laser butt welding of UHSS, mechanical properties was improved by using small laser beam diameter and Al-Si coating layer caused fracture of weld metal. In case of laser lap welding of UHSS, Al-Si coating layer resulted in formation of intermetallic compound at the fusion boundary where fracture occurred. Al-Si coating layer caused lowering mechanical properties of weld metal.