Free-piston linear generator (FPLG) engines currently gained great attention due to their capability to operate with variable fuel and compression ratio. This paper presents an experimental study on the effect of aspect ratio on the performance characteristics of the FPLG engine fueled by hydrogen. Three aspect ratios (i.e. 1.0, 1.5, and 2.0) are used to identify the engine combustion and performance parameters. The injection position is fixed in the middle of the stroke, while the equivalence ratio is kept at 1.0. The results indicate that the aspect ratio 2.0 produces the highest pressure, heat release, and shortest combustion duration. Whereas the aspect ratio 1.0 produces higher combustion efficiency and operating frequency. The piston speed decreases with the decrease in aspect ratio, which gives a negative effect on the indicated mean effective pressure and power output of the PFLG. Overall, the aspect ratio has a significant influence on engine performance characteristics. © 2020 Hydrogen Energy Publications LLC