Die-sinking electrical discharge machining (EDM) is widely used in the process of making die casting moulds, plastics moulding and forging dies. But low machining efficiency and poor surface quality of die- sinking EDM are the major drawbacks of this process that restricts its use in mechanical manufacturing. Therefore it is of great importance to improve the machining efficiency and surface quality of EDM. In this article, a surface roughness (SR) study on die- sinking EDM of SKD11 die steel has been carried out. Response surface methodology (RSM) has been used to plan and analyze the experiments. Current (I), pulse on time (Ton) and voltage (U) were chosen as process parameters to study the die- sinking EDM performance in term of SR. The results indicated that in order to obtain a low value of Ra within the work interval of this study, I and Ton should be fixed as low as possible, and conversely, the larger the selected U. And the optimal value of Ra was 0.44 m at optimal process parameters I = 1 A, U=60 V and Ton=9 s. The mathematical model for the SR can be effectively employed for the optimal selection of the die- sinking EDM process parameters. Empirical tests show that the model can calculate quite accurately predicted by SR. EDS, XRD and microstructure analysis has been completed to understand the form of white layer on the surface of the workpiece material. Machining surface quality at optimal parameters were analyzed. It has shown that resulting surface quality after die- sinking EDM is good.