The process of laser surface modification is a complex transient three-dimensional heat conduction problem. A moving heat source and a moving phase boundary further complicate the modeling. This general problem can be simplified using appropriate assumptions resulting in an energy balance equation used to derive a melt depth model as a function of interaction time and laser power input. The model can then be used to design and implement a real-time feedback control scheme. The measurement used for feedback to the control algorithm is the surface temperature. The real-time surface temperature measurements are obtained by using a unique pyrometer arrangement. This measurement scheme allows the pyrometer measurement aperture to directly follow the laser beam path through the entire surface modification process in real-time. Experiments using a Nd:YAG laser were performed on mild steel samples to verify the suggested model’s results. Introduction For a successful laser surface modification process such as surface hardening and remelting, the heat affected zone depth or the melt depth is an important quantity [1-4]. The processing results depend on those parameters and should be controlled in real-time during the process. But these quantities are immeasurable during the process. Instead we can measure the maximum surface temperature by using infrared pyrometer. An appropriate model is required to relate the surface temperature measurement to the melt depth. Previously, a one-dimensional melting model [2] was developed, but the difficulty in deriving the relationship between the melt depth and the surface temperature impaired its usefulness for real-time control applications. The proposed model calculates the melt depth based on the energy balance equation proposed by Pantelis and Vonatsos [3]. Once the melt depth is known, the surface temperature can be obtained from Xie and Kar’s model [4]. To verify the model, single-line scan experiments were performed on mild steel plates(SAE 1010 cold drawn). The samples were then cross-sectioned and melt depth measurements were taken. The results were compared with the data from the model simulations. Surface temperature measurements taken during the experiments were also compared with the simulation results.