Abstract: In order to improve measuring precision of thread based on machine vision, tangential and vertical projection measuring methods were discussed in theory, and verified experimentally, mainly related to the matter of extending helicoidal surface shade error when measuring thread profile with both the methods. After introducing the tangential and vertical projection measurement methods, the error sources were analyzed which cause principle error in measuring thread. The shade error calculating formula was given that caused from measuring thread with vertical projection measurement, and the effect that the shade error and the error caused by different lead angle was studied in measuring thread profile experiments. The result shows that the helix angle measurement error caused by extend helicoids shade error can reached over 1°when measuring larger-size helical pitch by vertical projection measurement, it can't be ignored for helix angle measurement.