This paper proposes an evaluation model for water quality based on fuzzy set theory comprehensive. Based on water quality analyzed data. Nine pollution factors were used: pH, TSS, DO, turbidity, NO2-N, NO3-N, NH3-H, COD, total Colifonn and evaluation grades based on guiding of the Decide No.879/QD-TCMT of VEPA The process of fuzzy comprehensive evaluation have five steps: (1) Establishing the V matrix (factor x controlling sample)
(2) selection of quality grading system by matrix U
(3) selection of method and calculation the weighting value for the factors contributing in evaluation
(4) calculation of the fuzzy matrix R for samples
(5) calculation of the value of vector of membership function to conclude the quality grading of sample of controlling. By the digitizing of pollution grading follow the natural language (Serious, heavy, medium, slight, not yet pollution), specifying the membership function by probability principle, after defuzzying the model allow conclusion of evaluation by natural language. This help the environmental controller to evaluate scientifically also as precisely the quality of waste water of the enterprises.