The aim of this paper is to design and optimize portable axial flux permanent magnet (AFPM) machines for human power generation. To achieve greater portability of a generator, we focus on minimizing active mass of the generator. The generator required is to deliver 60 W at different rated speeds. AFPM generators with concentrated windings and airgap windings are chosen as a generator type. Expressions to decide basic dimensions and parameters of the generators are derived in the paper. Next, optimum design of those generators is done to find the dimensions and parameters minimizing active mass at different speeds and different efficiencies. Based on the results of the design, a generator with concentrated windings (723 g) is heavier than a generator with air-gap windings (TORUS machine, 375 g) at 4000 rpm and 90% efficiency. At 2000 rpm and 80 % efficiency, the generator with concentrated windings (367 g) is lighter than the generator with air-gap windings (419 g).