Laser transformation hardening(LTH) is one of the laser surface modification processes.The surface hardening of rod-shaped carbon steel(SM45C) was performed by lathe-based laser composite processor with Gaussian-beam optical head.The LTH characteristics by dominant processes,longitudinal and depth directional hardness distributions and behaviors of phase transformation in hardened zones were examined.Especially,two concepts of circumferential speed and theoretical overlap rate were applied.When laser power increased or circumferential speed decreased,the surface hardening depth gradually increases due to the increased heat input.Moreover,the longitudinal hardness distribution particularly shows periodicity of repetitive increase and decrease,which results from tempering effect by overlap.Finally,the feasibility of laser transformation hardening is verified by using the beam with Gaussian intensity distribution. Laser transformation hardening (LTH) is one of the laser surface modification processes. The surface hardening of rod-shaped carbon steel (SM45C) was performed by lathe-based laser composite processor with Gaussian-beam optical head. LT LT characteristics by dominant processes, longitudinal and depth directional hardness distributions and behaviors of phase transformation in hardened zones were examined .Especially, two concepts of circumferential speed and theoretical overlap rate were applied .When laser power increased or circumferential speed decreased, the surface hardening depth gradually increased due to to increased heat input. Moreover, the longitudinal hardness of repetitive increase and decrease, which results from tempering effect by overlap. Finally, the feasibility of laser transformation hardening is verified by using the beam with Gaussian intensity distribution.