We have generated Ar plasma in dense plasma focus (DPF) of hypocycloidal pinch (HCP) for extreme ultraviolet (EUV) lithography and investigated an emitted visible light for electro-optical plasma diagnostics. We have applied an input voltage 4.5 kV to the capacitor bank of 1.53 uF, and Ar gas has been filled into a diode chamber. The EUV emission signal whose wavelength is about 6-16 nm has been detected by using a photo-detector (AXUV-100 Zr/C, IRD). It has been found that the EUV intensity was decreased as the Ar pressure was increased. For measurement of the electron temperature in plasma pinch for HCP, we have measured the Ar spectrum lines for various Ar pressures with spectrometer (HR4000CG, Ocean Optics). If we assumed that the focused plasma regions satisfy the local thermodynamic equilibrium (LTE) conditions, the electron temperature of the hypocycloidal pinch could be obtained by Boltzmann plot method of optical emission spectroscopy (OES). The electron temperature has been approximately observed to be 2.6 eV at the Ar pressure 1 mTorr. The electron temperature and EUV intensity have been measured in accordance with the Ar pressures. It has been shown that the optimized Ar gas pressure is at 1 mTorr for the intensive focused plasma. (C) 2011 Elsevier B. V. All rights reserved.