Laser welding of stainless steel and mild sheets is considered and thermal efficiencies pertinent to welding are formulated. The weld bond mass ratio (alpha), weld mass loss ratio (beta), heat loss ratio (gamma) and mass fraction (delta = m(Ti)/m(tot)) are defined when formulating the first and second law efficiencies. The lump parameter technique is considered in the analysis. It is found that the first and second law efficiencies increases with weld bond mass ratio while it reduces with increasing weld mass loss ratio, heat loss ratio, and mass fraction. This reduction is sharp with increasing weld mass loss ratio.