Whilst the interest of industry for a second generation (post-Concorde) large supersonic transport seems to be, today, very limited, several pre-industrial research projects for a smaller civil supersonic aircraft have been conducted over the last fifteen years. While flying at supersonic speed, aerodynamic pressure perturbations produced by the aircraft in the close vicinity will propagate through the atmosphere down to the ground, leading to a sonic-boom (Figure 1). One of the challenges to be overcome to ensure the economical viability of a future supersonic civil transport is certainly the capability to fly supersonically overland. This capability will require that the aircraft generates an "acceptable" sonic boom, meaning a sonic boom which produces, at ground level, no (or sufficiently reduced) annoyance to humans and animals. The accurate modelling and prediction of the ground sonic boom signature is therefore a key-point for the design of a new civil transport supersonic aircraft and its future certification. Within the 7th Framework Program European project HISAC, an acronym for HIgh Speed AirCraft, ONERA has been applying its in-house developed sonic boom prediction methodology (Figure 2) to evaluate the effect of some aircraft geometry parameters on the characteristics of the ground sonic boom produced by an aircraft configuration representative of a small size supersonic transport aircraft. The ONERA sonic boom prediction approach is based on three components. First the near field inviscid CFD calculation of the aircraft in supersonic cruise conditions is performed using elsA solver. Then the CFD-aerodynamic pressure field is extracted on a cylinder surrounding the aircraft and the matching between aerodynamic and aero-acoustic propagation theories is obtained thanks to a multipolar decomposition approach . Finally, a non-linear acoustic propagation code (TRAPS) is used to propagate the sonic boom through the atmosphere, from the flight level down to the ground. The accuracy of sonic boom prediction is highly depending on the near field shock capturing capability of the solver. The impact of mesh refinement (Figure 3) and the interpolation process have been studied on a simplified body-only configuration. In order to identify key-parameters to minimize the sonic boom intensity, the previously presented prediction methodology has been used to perform a parametric study of the effect of several aircraft geometry parameters, such the number of lifting surfaces, dihedral of the lifting surfaces or the shape of the aircraft nose (Figure 3).This study helps to understand the physical phenomena by which the resulting sonic boom shape is linked to the geometry of the aircraft and therefore provides useful guidelines for the design of a low-boom aircraft configuration.