The cellulolytic fungus Trichoderma reesei is a potent cellulase producer and, therefore, cellulase hyper-producing mutants have been developed. However, morphological feature has still remained to be analyzed for understanding phenotypic change of T. reesei mutants. In this review, we show an electron microscopic observation of T. reesei to obtain new insights of morphological phenotypes of T. reesei mutants. We also successfully reconstructed the three dimensional structure of T. reesei hypha by using focused ion beam SEM technique.