In the present study the conductivity probe technique has been adopted to investigate slug flow in a vertical narrow pipe of 0.012m ID. Measurements have been made for air phase velocity varying from 0.05 to 7m/s and water from 0.15 to 2m/s. Several analyses of the random signals, namely PDF, FFT, and wavelet multi-resolution of the signal, have been performed. Different quantitative measures of the PDF and FFT curves as well the fluctuations at the different scales of the wavelet analysis have been used to identify the range of existence of the slug flow pattern. An objective flow pattern indicator has been developed on the basis of the measurement. The flow pattern indicator is effective in defining opureo and obubblyo slug flow and differentiating them from dispersed and annular pattern.