Fluid shear stress (FSS) acting on the apical surface of endothelial cells (ECs) can be sensed by mechanosensors in adhesive protein complexes found in focal adhesions and intercellular junctions. This sensing occurs via force transmission through cytoskeletal networks. This study quantitatively evaluated the force transmitted through cytoskeletons to the mechano-sensors by measuring the FSS-induced strain on SFs using live-cell imaging for actin stress fibers (SFs). FSS-induced bending of SFs caused the SFs to align perpendicular to the direction of the flow. In addition, the displacement vectors of the SFs were detected using image correlation and the FSS-induced axial strain of the SFs was calculated. The results indicated that FSS-induced strain on SFs spanned the range 0.01-01% at FSSs ranging from 2 to 10 Pa. Together with the tensile property of SFs reported in a previous study, the force exerted on SFs was estimated to range from several to several tens of pN. (C) 2010 Elsevier Inc. All rights reserved.