In this study, a rule-based Mamdani-type fuzzy logic (RBMFL) model was developed for prediction of compressive strength of lightweight concretes containing silica fume (SF) and fly ash (FA). Pumice was used as the aggregate in the concretes. In the concrete mixture 0, 5, 10, 15 and 20% of fly ash and 0, 5, 10, 15 and 20% of silica fume, for each value of fly ash content, were added by replacing the cement. The compressive strength of the lightweight concretes was investigated experimentally. Experimental results were used to construct the fuzzy logic model. In the study, the values obtained from the model and experiment were divided into five groups (each group has five experimental results), according to the FA and SF contents, to evaluate approximate reasoning ability of RBMFL model. As a result, RBMFL model has shown satisfying relation with experimental results, which suggests an alternative approach to evaluation of compressive strength of lightweight concretes containing silica fume and fly ash.