Helium plasma containing a small amount of O-ions was produced by the linear Z-pinch discharges in He–O 2 (0.5 mol%) mixture, and the temporal changes of emission intensities of the OII to VI-ion lines as well as of the Hell 304 Å resonance line were measured in the extreme ultraviolet λ100 to 550 Å. At the same time the temporal change of the electron density N e was measured with a laser interferometer and the temporal change of the electron temperature T e was determined from that of the Hell free-bound intensity distribution. Under the measured N e and T e changes, the simultaneous rate equations for the ionization of O-ions have been calculated to give the temporal changes of the OII to VI-ion line intensities. The result falls far from the observation, especially the calculated intensities of OV- and VI-ion lines are much smaller than the observed intensities. The ionization processes of impurity O-ion in the He plasma are discussed.