This paper presents an investigation into the drilling of micro-holes of diameter 0.3 mm in tool steel H13 by means of the Micro-EDM process. Scanning Electron Microscopy (SEM) and Scanning White Light Interferometry (SWLI) techniques are used to determine the influence of the drilling process parameters upon the surface roughness. The results reveal a series of randomly overlapped craters upon the machined surface, which represent the position and chronological sequence of individual sparks during the machining process. As the voltage and current increases, the crater appearance changes from a conical to a cylindrical shape, and its depth and diameter both increase. It is shown that surface roughness deteriorates as the pulse voltage increases. Finally a regression equation is established between the diameter of the crater and the pulse voltage, pulse current and pulse-on duration parameters. This equation enables the diameters of the craters, and hence the surface integrity of the machined surface, to be predicted for a given set of process parameters, and is therefore a valuable tool in achieving the goal of precision machining.