Inter-laboratory comparison tests of sustainable current in AC NbTi composite superconducting wires have been carried out at five organizations on a tentative test method. It was found that fluctuation of the measured sustainable current defined by the power criterion was ±1% for intra-laboratory tests and in a range of ±5%~±20% for inter-laboratory tests. The measurement results showed that the longitudinal component of the AC external field influenced the AC sustainable current. This paper describes the test results of the AC sustainable current and discusses on the standardization of the AC sustainable current measurement for the AC wires.