This paper presents a semi-analytical method for the weight estimation of fuselages with an oval cross-section, applied to blended wing body aircraft. The weight estimation of the fuselage primary structure is based on a structural analysis of two-dimensional crosssections and it is completed by a set of empirical relations for the weight estimation of the secondary structure and non-structural items. A conceptual design study on a stable (5% static margin), 400-passenger blended wing body aircraft has been performed with the weight estimation method implemented in an aircraft conceptual-design program. A top level requirement for the harmonic range of 15,200km was imposed on the design. From the anaylis a maximum take-off weight of 395 metric tons has been found, with an operative empty weight fraction of 45% and a fuselage airframe weight of 47.3 metric tons (amounting to 26% of the operative empty weight). The results of this study are presented in comparison to the Boeing 777-200LR airliner, showing a slightly higher operative empty weight fraction, a lower fuel consumption of 2.05kg per passenger per 100km.