The ion temperature in a large diameter plasma is measured with the high-resolution optical emission spectroscopy. It is found that the ion temperature is low when the radial distribution of the plasma is flat profile compared with that in the case of peaked profile. The results of numerical calculation carried out by using the hybrid model agree well with the experimental results. It is found that the calculated ion temperature increases as the radial electric field increases. Thus, it is concluded that the decrease of the ion temperature is due to the decrease of the radial electric field. (c) 2005 Elsevier B.V. All rights reserved.