Heat engines are known for high power and energy densities compared to chemical batteries. The availability of free thermal energy —solar and ‘waste heat’ from electronics provide an opportunity for power generation at miniature length scales (millimeter). Though free piston based expanders at macro length scales (centimeter and above) are found to be suitable for heat energy harvesting, their implementation at miniature scales are challenged by significant parasitic losses such as heat loss and pumping work, yielding low thermal efficiencies. In this manuscript, through physics-based models the behavior and performance of a miniature free piston expander that operates on an open cycle was investigated. Here, the design space of the free piston expander with an objective to achieve an efficiency of at least 15% was explored. Three observations are reported that contribute to higher efficiency operation: (1) a higher injection pressure; (2) an optimum nondimensionalized duration of injection time of 1.5; and (3) softer springs, lower loads, and heavier pistons. A sample calculation showed that a centimeter-sized expander can generate an output power of 2.24 W at 18% efficiency. This study shows that both the performance parameters of the expander, namely efficiency and output power are sensitive to injection pressure of the working fluid compared to the time duration of the working fluid injection. The study reveals that the miniature free piston expander is promising for low temperature waste-heat harvesting.