Recently, metrology in the fields of synchrotron- and X-ray optics entered the nano-accuracy and sub-50 nrad rms range. In addition to developing novel surface profilers, now dedicated specific measurement technologies play very important roles in reducing errors in measurements. All facts, producing an error of about 10 nrad rms, must be treated very carefully. A temperature stability of 0.01-0.02°C (P-V) over 24 hours is one of most important parameters in ensuring nano-accuracy. The Elcomat 3000/8 autocollimator has large saw-tooth error of 269 nrad rms, which must be suppressed. A fixed reading location setting method shuns the saw-tooth impact, so that it is possible to reach sub-50 nrad rms accuracy in measuring a precise plane mirror. A dense measurement method, combined with finding a peakvalley center to remove saw-tooth effectively promotes accuracy for sphere test. In this paper, we detail a graphic method of combining multiple FW/BW scans in selecting stable files that can reduce the error by10-15 nrad rms.The alignment of precise parallelism between the autocollimator’s axis and the direction of the slide’s movement are introduced, and some nano-accuracy tests are introduced.