Abstract: In this study, Portland cement replaced with trass as a ratio of its weight of 0, 10, 15, 20, 25 and 30. By this procedure, six different cements were prepared and effect of trass was investigated on the compressive strengths of the cement mortars at 2, 7 and 28 days. Also, by using Fuzzy Logic method, a prediction model was formed based on the quantity of trass to predict the compressive strengths of cement mortars which could not be determined with experimental approaches. The experimental results are compared with the fuzzy logic results. As a result, the correlation coefficient for the compressive strength of the cement mortars at 2, 7 and 28 days is found 0.97. This result shows that the developed model can be successfully applied in the cement sector.