This paper deals with a systematical evaluation of the effect of joint preparation on LASER butt weld quality of structural steel for automobile application. Test results demonstrated that the sound weld could be obtained only by proper combination of welding conditions. Permitted joint gap tolerance and offset for a defect free weld in the autogenous LASER butt welding of 3.2㎜ thick structural steel is about 0.3㎜ and 0.3∼0.4㎜, respectively. However, the gap and offset should be maintained as small as possible. When the joint gap and offset, are greater than a critical amount, weld bead becomes irregular and forms a concave ditch on the weld surface with a poor penetration. In case of the offset values were greater than 0.5㎜, unmelted zone, which would act as a root crack in service. was also detected.