Complex turbulence field around a flat paddle impeller in a stirred vessel was investigated experimentally with LDA measurement and phase averaging analysis. The relative mean velocity filed against the flat paddle was obtained and detailed examination was made on the production mechanism of Reynolds shear stress. Containing inflexion point, the relative velocity profile in downstream direction evolves like separated shear layer. Reynolds shear stress profile has a large peak, however, the location unnecessary coincides that of the maximum mean velocity gradient. There is the case that the large Reynolds shear stress occurs around boundary between inflow and outflow behind the edge of flat paddle. It is expected that quasi-periodic inflow and outflow contribute Reynolds shear stress production.