A fundamental understanding of the mechanical behavior of the indium tin oxide (ITO) layer is very important because cracking and delamination of the ITO layers have been a critical obstacle for mechanically robust flexible electronics. In this study, the intrinsic mechanical properties of ITO thin films without a substrate were measured by utilizing a freestanding tensile testing method. Young's modulus (89 +/- 1 GPa), elongation (0.34 +/- 0.02%), and tensile strength (293 +/- 13 MPa) of amorphous as-deposited ITO thin films were successfully measured. The sheet resistance, transparency, and thickness of the as-deposited films were 32.9 +/- 0.5 Omega/sq, 92.7% (400-700 nm), and 152 +/- 6 nm, respectively. First, we investigated the effects of annealing temperature on the mechanical properties of ITO thin films. For 100- and 150 degrees C-annealed ITO thin films, which were amorphous, Young's modulus, elongation, and tensile strength were enhanced by increasing the packing density and reducing the structural defects. For 200 degrees C-annealed ITO thin films, which were polycrystalline, Young's modulus was further increased because of their highly packed crystalline nature. However, there was a significant decrease in elongation and tensile strength because grain boundaries act as critical defects. Next, the annealing time was varied from 0.5 to 6 h for a better understanding of the effects of the annealing time. As a result, the maximum elongation (0.54 +/- 0.03%) and tensile strength (589 +/- 11 MPa) were obtained at 150 degrees C for 1 h. Annealing for 1 h was appropriate for sufficient defect reduction; however, excessive annealing for more than 1 h increased the degree of partial crystallization of the ITO thin films. The proposed annealing conditions and the corresponding mechanical properties provide guidelines for the optimum annealing process of ITO thin films and quantitative data for mechanical analysis to design mechanically robust flexible electronics.