Abstract: The morphological change of Penicillium chysogenum during the penicillin fermentation cycle was studied. The shear environment in the fermentor was found to influence microbial morphology and to be closely related to the mean pellet diameter and effective length of mycelium during the middle and late periods of fermentation. Based on flow field test results in laboratory-scale stirred vessels, a number of parameters for describing the total shear environment in the fermentor were evaluated for their reliability, with the morphological change of P. chrysogenum being used as contrast. It was found that the morphological response to high shear environment exhibit an "impression-attenuation" mechanism, from which a concept of characteristic shear power can be deduced. An appropriately high shear environment is required for penicillin fermentation. With the concept of characteristic shear power, the design and operation of hydrofoil impeller can be implemented to provide a proper shear environment for microbial growth and metabolism for synthesis of the object product.