A modular implant system allows a surgeon to secure an implant assembly to thee diaphysis of a long bone. The system includes a set of anatomically-designed diaphyseal fitting modular implant components. One end of each diaphyseal implant component is a Morse taper post for connection to another implant component such as a modular articular component, a segmental component or an intercalary component. The other end of each diaphyseal component is a tapered porous surface. In some sizes, the tapered porous surface includes a plurality of steps. The tapered porous surface is received with a tapered bore in the bone diaphysis that is prepared to match the size and shape of the tapered porous surface. The diaphyseal implant is easy to insert and remove, does not bind before fully seating, is designed to prevent stress shielding and provides the surgeon with a host of stem options with its modularity.