The bi-modular hip prosthesis is characterized by two tapered connections: a circular cross-section at the head-neck interface and a rectangular cross-section at the neck-stem interface. Even if the latter guarantees customization, it concerns a high rate of early failure. The connection resistance is relatable to machining (tolerances cause angular mismatch), implantation (hammering force or manual), and usage (Body Mass Index [BMI]). Due to the lack of literature about the neck-stem coupling, this work aims to investigate how the geometry of the rectangular taper connection and the external loads affect the fatigue strength of a bi-modular hip prosthesis through a 3D Finite Element Model (FEM). Nine combinations of neck-stem coupling are obtained considering the tolerances' limits on frontal and lateral angles as