Mass transfer and shear force have significant effects on nemadectin production by Streptomyces cyaneogriseus ssp. noncyanogenus. They are always the conflict-ridden problems in nemadectin fermentation process. In this study, the flow field characteristics under different impeller combinations were quantitatively evaluated in 5 L stirred-tank bioreactor through the laser particle image velocimetry (PIV) system. Results demonstrated that the radial-axial impeller combinations with the time average velocity at 0.38-0.54 U-tip, the turbulent kinetic energy dissipation rate at 6.4-10.6 e/(ND2)-D-3, and the shear stress rate was 40-150 s(-1), were more conducive to cell growth, nemadectin biosynthesis, cell's activity, respiratory metabolism than other combinations. The highest nemadectin yield was evaluated up to 1543.3 +/- 18.5 mu g/mL, which was 31.68 % higher than that of the radial flow impeller combinations. This study provided the important guideline for the selection impeller combinations' on large-scale nemadectin production.