In this study, microstructure, mechanical, and conductivity characteristics of Hardox 400 steel were changed with different heat treatments and effect of such characteristics on its machinability with Wire Electrical Discharge Machining (WEDM). Optical microscope examinations were performed to determine various characteristics, and additionally microhardness and conductivity measurements were conducted hereof. L18 Taquchi test design was conducted with three levels and four different parameters to determine the effect of such alterations on its machinability with WEDM and post-processing Cutting width (kerf), Surface roughness (Ra), Material removal rate (MRR) values were determined. Micro changes were ensured successfully by using applied heat treatments. The best kerf value was obtained from sample 6 which was tempered from 350 degrees C. The microstructure of this sample was composed of commonly alpha ferrite and few pearlites. The best Ra value was obtained from sample 3 which was tempered from 200 degrees C. The microstructure of this sample was composed of few a ferrite and commonly pearlites. The best MRR value was obtained from sample 5 which was tempered from 300 degrees C. The microstructure of this sample was composed of almost equally alpha ferrite and pearlites. Additionally the most effective parameters on Ra were determined as hardox and current. The most effective parameters on kerf and MRR were determined as time off and hardox.