This paper presents a type of energy conversion device, a free-piston linear generator (FPLG) engine which can generate electrical energy when a free piston engine (FPE) is coupled to a generator.An FPLG engine prototype is designed, fabricated, and developed to operate both in combustion and expansion mode at the Center for Automotive and Electric Mobility (CAREM).The generator unit consists of different components.Stator core is one of the components which is known to contribute to the generating capacity of the engine.So, experimental tests were conducted using two stator core materials - Perspex stator core, GU(P) and metal stator core, GU(M) materials, to study the material effect on the engine performance.The operating pressure of the working fluid, compressed air, was maintained at 8 bar pressure, while 33.3 Hz and 66.6 Hz were the operating frequencies set for the intake valve opening and closing respectively.Based on the experimental data, the motion characteristics and output performance of the engine were discussed.The results shows that the stator core materials have effect on engine performance.Compared with the other material, GU(M) material is found to have a shorter translator stroke and higher cogging force.However, translator velocity that resulted at the operating condition is higher with GU(P) than with the GU(M).The peak power and indicated efficiency were found to be 423.4 W and 64.2, respectively when GU(M) material is used.Therefore, the power generation of an FPLG engine can improve if suitable stator core material is chosen for the generator unit. © 2023, Institute of Technology PETRONAS Sdn Bhd.