  We present a study of the optical spectral properties of 115 ultraluminous
infrared galaxies (ULIRGs) in the southern sky. Using the optical spectra
obtained at CTIO 4 m and provided by the 2dF Galaxy Redshift Survey and the 6dF
Galaxy Survey, we measure emission line widths and fluxes for spectral
classification. We determine the spectral types of ULIRGs with H_alpha
measurement using the standard diagnostic diagrams. For ULIRGs without H_alpha
measurement, we determine their spectral types using the plane of flux ratio
between [OIII]_lambda5007 and H_beta versus [OIII] line width based on our new
empirical criterion. This criterion is efficient to distinguish active galactic
nuclei (AGNs) from non-AGN galaxies with completeness and reliability of about
90 per cent. The sample of 115 ULIRGs is found to consist of 8 broad-line AGNs,
49 narrow-line AGNs, and 58 non-AGNs. The AGN fraction is on average 50 per
cent and increases with infrared luminosity and IRAS 25-60 micron colour,
consistent with previous studies. The IRAS 25-60 micron colour distributions
are significantly different between AGN and non-AGN ULIRGs, while their IRAS
60-100 micron colour distributions are similar.
