In general, self-standing polymer ultra-thin films are difficult to be handled, and therefore their mechanical properties have been poorly understood. We carried out the uniaxial tensile test of ultra-thin films floating on the surface of water and measured the stress-strain curves of polystyrene (PS) ultrathin films with thickness of around 100 nm. We found that in the stress-strain curves of PS ultra-thin films, yielding points similar to ductile materials appear. We also employed Brewster's angle reflection imaging to follow the visual appearance of ultra-thin films during tensile tests. We found that the narrow shear deformation zones (SDZs), which are 2D analogue of crazes, appear at the yield point, and both the yield and equivalently crazing stresses decrease with decreasing the film thickness. Moreover, the reduction of crazing stress is more significant for the higher molecular weight polystyrene. (C) 2017 Elsevier Ltd. All rights reserved.