Abstract: This paper tries to prove theoretically and practically that the current widely used mode of weighting in levelling nets adjustment is quite inappropriate.It requires more work done and reduces the efficiency of adjustment on the contrary.Basing on the mechanism of errors in levelling.a rigorous first derived for calculating the mean accidental error per kilometer η.And thence by the help of data of several repeating levellings,an analys is further made for the characteristic of the mean systematic error per kilometer σ.As σ is practically yet undeterminahle for the cuent levelling practice,cases of different modes of weighting with σ neglected are studied.The effect of these approximations of weighting on the efficiency of final adjustment is estimated and tested in actual examples.The result reveals that the currently used mode of weighting is the worst.Better ones are suggested.