The effects of pre-heat treatment(Q/T) on microstructure and hardness of SCM435 structural steel nitrided by micro-pulse plasma was investigated. The quenching and tempering temperatures for obtaining matrix hardness of SCM435 steel on range of HRC30 to HRC40 desired for machine parts were about 860℃ and 500℃ respectively. The case depth of SCM435 nitrided at 480℃ for 5 hours was independent of pre-heat treatment condition and was approximately 150㎛. However, hardness and compactness of nitrified layer on Q/T treated specimen were more heigher than annealed specimen. The case depth increased linearly with the increase of nitriding temperature, however, the hardness of nitrified layer decreased with the temperature. Phase mixture of γ` -phase(Fe₄N) and ε-phase(Fe₃N) were detected by XRD analysis in the nitrified layer formed at optimum nitriding condition, and only single γ`-phase was detected in the nitrified layer formed at higher nitriding temperature such as 540℃. The optimum nitriding temperature was approximately 480℃ which is lower than tempering temperature for preventing softening behavior of SCM435 matrix during nitriding process and the surface hardness of nitrified layer obtained by optimum preheat treatment condition was about Hv930