An international round robin test was promoted to establish a test method for room temperature mechanical properties of commercial REBCO superconductive tapes. Seven laboratories practiced a tensile test under the direction of guideline REBCO13 for four different kinds of REBCO tape. From the stress versus strain curve, the modulus of elasticity and the 0.2% proof strength were measured. The scatter of measured values was analyzed by evaluating the RSU (relative standard uncertainty). To judge the major contribution to scattering, an F test was applied. The major source of RSUs was estimated to be the influence of inter-laboratory scattering. In order to reduce the overall scattering, it is suggested that the REBCO13 guideline should be modified with respect to the following three experimental factors. The window determining acceptable data should be narrowed, and the initial strain rate should be suppressed to less than 1 × 10−4 (s−1). Repeated thickness measurement is recommended to reduce the standard uncertainty.