This paper describes how to use radio signals during machining on Wire EDM to distinguish machining conditions. From experimental results it was found that radiation in stable state occurs more than one in unstable. Hence, a detectivemeter on machining conditions is proposed for a operator to catch the condition. Accordingly, peak frequencies of radiation are precisely checked and a relation of radio signals to removal rate is quantitatively discussed. These check and discuss are also useful to prove radio-signal sensor not only for distinguishing but also for optimization of machining conditions. In consequence it was showed that radio signals have more application in the future.