Abstract: By analyzing concept of blunders, this paper points out that blunders are fuzzy concept. Fuzzy concept can not be simply described by ordinary set. It only can be described by fuzzy set. Therefore, based on the fuzzy logical relationship between observation errors and residuals the membership function of blunders has been presented. At last the algorithm of robust estimation considering fuzzy logical relationship has been designed. It is shown that the robust estimation considering fuzzy logical relationship is more effective to repress the influence of blunders and more correct to locate blunders.