Abstract: This paper presents a formulation of the back analysis using the fuzzy finite element method. The methodcombines the conventional back analysis with fuzzy finite element method to consider the uncertain,imprecise,orfuzzy nature of back analysis problems in geomechanical engineering. We also suggest the calculating formulae aboutthe membership function,the weight of contribution,the reliability for back analysis results,and a new method toconsider the fuzziness of element number.The fuzzy analysis of the membership function, the sensitivity factor,thefuzzy measure,and the reliability for fuzzy back analysis results are illustrated with an example.