Abstract The composite of tungsten carbide–cobalt (WC–Co) is a very important material used in carbide dies, cutting tools and components under erosion. The machining of this material is very difficult because of its very high hardness. Among conventional methods, this material can be machined with pendulum grinding method when special and expensive discs were used. However, the material removal rate is very low and other limitations in machining parameters and workpiece shapes are imposed. Among non-traditional methods, electro discharge machining (EDM) is considered as the best candidate for machining this material. However, the process is unstable especially when die sinking machines are used. This is due to generating open circuit, short circuit and arcing pulses. This paper analyses the instability of EDM process when machining WC–Co composite and introduces the ways to control it.