CP steel was developed to reduce the weight and increase the strength of car body. When it was welded using state-of-the-art disk laser welding, the effected of boron on the microstructure and hardness were investigated. Welding power was fixed at 3.5kW and welding speeds were 4,8 and 12m/min. Full penetration occurred in welding speed of 12m/min and weld bead was almost unchanged with boron contents. But the welding speed increased, the upper and lower bead were narrowed. In a welding speed of more than 8m/min, underfill defects were formed on the bead bottom. The hardness of weld zone was somewhat fluctuation in fusion zone and HAZ showed the highest hardness values. The hardness of each region showed little change with the boron contents, and softening phenomenon occurred in the HAZ near the base metal regardless of the boron contents.