Interfacial phenomena between liquid iron and solid CaO is important due to the extensive use of solid CaO in steelmaking processes. In this study, the dynamic change in surface tension and contact angle of liquid iron (Fe-4mass%C-S) on CaO substrates during desulfurization was investigated at 1 623 K using a sessile drop technique with varying initial sulfur content (0.002-0.514 mass%). In addition, an electron probe microanalysis (EPMA), a field emission scanning electron microscope (FE-SEM) and an atomic force microscopy (AFM) were applied to investigate the interface between metal and substrates and the surface of the solid substrates. The surface tension increased rapidly just in a few minutes, and then approached an easy constant slope in every experiments. The contact angle decreased considerably with time for most samples, approaching a constant value. Exceptionally, the sample initially containing 0.002 mass% sulfur showed just slight decrease in contact angle. Based on experimental results, the dynamic reactive change in the work of adhesion of liquid iron on solid CaO was computed. The work of adhesion was increased by both the increase in the surface tension and the decrease in the contact angle. Especially, the change in the work of adhesion in the initial stage of desulfurization was considerable.