In the literature, the results of numerous studies were used to propose predictive tools for the evaluation of fragmentation efficiency in rock mass blasting. The methodology developed during several years at the Centre de Géologie de l’Ingénieur is based on the comparison of rock mass and corresponding muck piles block size distributions [1–4]. This paper introduces a new approach of the evaluation of the part of blasting energy used in fragmentation, based on the assessment of the energy devoted to the formation of crack surface. The fundamental assumptions of this method will be exposed and discussed in detail. The method is then applied to blasts performed in two Spanish mines in different geological contexts. The corresponding results are presented and the influence of some blast parameters (such as geological rock mass structure, specific energy and time delays) is discussed.