For this study, a test rig of free piston expander-linear generator (FPE-LG) for organic Rankine cycle (ORC) waste heat recovery system is built. Based on the free piston expander-linear generator test rig, an experiment of external load resistance effect on the motion characteristics, indicated efficiency, output performance and energy conversion efficiency of the free piston expander-linear generator is conducted. The displacement and velocity of the free piston assembly increase with the external load resistance. The indicated efficiency of the free piston expander (left cylinder) decreases with the external load resistance. As the external load resistance and intake pressure increase, the peak voltage output of the free piston expander-linear generator increases. The maximum peak voltage output is 44.4 V when the intake pressure is 3.0 bar, the operating frequency is 1.5 Hz and the external load resistance is 40 Omega. The peak power output of the free piston expander-linear generator initially shows a non-linear relation increase with external load resistance and tends to stabilize in the end. When the intake pressure is 3.0 bar, the operating frequency is 1.5 Hz and the external load resistance is 30 Omega, the maximum peak power output of 110 W can be achieved. The energy conversion efficiency of the free piston expander-linear generator increases gradually with growing of the intake pressure and external load resistance until the energy conversion efficiency reaches the maximal value, and then the energy conversion efficiency decreases with the intake pressure and external load resistance. The maximum energy conversion efficiency can reach up to 73.33%.