Abstract: This paper aimed to study the developed cocurrent upflow and downflow gas／solids systems in vertical pipes using deterministic chaos theory.Fluctuation signals which reflect the density pulsation in systems were obtained by an optical fiber density probe.The effects of gas velocity and solids flux were investigated and the differences of local chaos characteristics between upflow and downflow systems were compared.The result shows that chaos parameters can be used to describe the radial structure of gas／solids wto phase flow in the fully developed regime of vertical pipes.The local D_(ml) and K_(ml) are in the same range of magnitude in both riser and downer and are strongly correlated with the local solids fraction,which implies that the solids fraction is a key factor of influencing the turbulent structure in twophase flow systems and must be considered carefully in modeling the riser and downer.