Abstract: In this paper, an undetectability index (UI) for measurement error was proposed by using the projection matrix. If the UI value of the measurement error is higher, there may be larger difference between the measurement error and actual error, which means the omission of gross error may easily happen for the measurement with higher UI value. Hence, this paper utilized the UI value to modify all the measurements errors，which would further be adopted in the detection of gross error so as to improve the accuracy of the gross error detection. The simulation results on power system show that the modified gross error detection can attain higher accuracy of the gross error detection.