This paper investigates the effect of initial residual stress and pre-strain on residual stresses due to laser shock peening for Alloy 600 using numerical simulation. For simulation, the strain rate dependent Johnson-Cook hardening model with a Mie-Grüneisen equation of state is used. Simulation results are compared with published experimental data, showing good agreement. It is found that the LSP process is more effective for higher initial tensile residual stress and for larger initial pre-strain in terms of compressive stress at the near surface. However, the effective depth decreases with increasing initial tensile residual stress and initial pre-strain.