Free-piston machines are widely used in refrigeration compressors, cryocoolers, expanders, and combustion engines. Without crank mechanism, the free-piston machines are more efficient. However, the piston position of free-piston machines needs to be accurately detected with cost-effective methods. This article proposes a novel sensorless stroke detection technique using a low-cost inductive coil wound around main core for resonant free-piston machines. The flux linkage based sensorless technique was implemented in a moving-magnet-type linear compressor. A magnetic equivalent circuit (MEC) model and a finite-element analysis (FEA) model are presented to form the theoretical basis. The flux linkage in each core varies linearly with the piston displacement and a good agreement is shown between the MEC and the FEA models. Experiments are conducted in the linear compressor at resonance to prove the sensorless concept and to validate the two models. The flux linkage difference at two zero-current crossing points is shown to be a linear function of the piston stroke. The average percentage error between the measurements and the MEC model is about 12%. The proposed low-cost sensorless stroke detection technique can achieve an error of only 4%. The adoption of this novel technique is crucial to the commercialization of free-piston machines for high efficiency.