The flow patterns of oil/water two-phase flow in vertical upward pipes were characterized by the analysis of symbolic time series based on the conductance fluctuating signals.The study showed that the symbolic sequence temporal irreversibility Tfb and chi-square χ2fb statistics had little change with oil-in-water flow pattern variations for water cut (Kw) ranging from 61% to 91% but showed irregular sudden changes with transitional flow pattern variations for water cut 51%.When distinguishing the transitional flow pattern from oil-in-water flow pattern

the symbolic time series analysis method presented more unique characteristics and was a useful assistant diagnostic tool for the identification of oil/water two- phase flow patterns.