Abstract: In the wind tunnel balance calibration system,the main function of the load adapter is to ensure that the application point’s location coordinates are accurate.However,because of the manufacture error and dimension measurement error,the application points’real locations are drifted off from the relevant right locations in the theory,which brings errors to the applica-tion point’s location coordinates.These errors would be transferred to a balance calibration for-mula by a load adapter,which affects the accuracy of the loads measured by the balance.There-fore,it is necessary to research on the evaluation and expression of the uncertainty for a load a-dapter.Firstly,according to GUM (guide to the expression of uncertainty in the measurement, ISO/IEC GUIDE 98-3:2008),a method and steps for uncertainty assessment of a load adapter are founded.Then an expression and technical indices of the uncertainty for a load adapter are given.Finally,taking the type A load adapter belonging to the balance calibrated rig (code:TJZ-1 )for example,detailed evaluation processes and results about the uncertainty are given.The e-valuation results of the uncertainty indicate that the mean value of the standard uncertainty of all application points is 0.044mm;the mean value of the relative expanded uncertainty of all mo-ment’s arms is 0.0072%.