We investigated theoretically the electronic control of spin-alignment in pi-conjugated molecular magnets, stimulated by the recent successful observation of the spin control in novel diradical molecules. To clarify the mechanism of the spin alignment, a microscopic model was designed for a pi-electron system interacting with two stable radical groups. We demonstrated that electronic doping induces a transition from low-spin to high-spin states, depending on topological structures of molecules and the strength of interaction. In the doped case, spins align in a different way from that predicted by the topological rule established in the half-filled case. These theoretical results provide useful insights into molecular design of organic magnets controllable by external stimuli. (C) 2004 Elsevier B.V. All rights reserved.