The thermal efficiency of a differential stroke engine based on the Atkinson cycle is higher than that of engines with an Otto cycle or Diesel cycle. To fulfill the requirement of the expansion ratio that is greater than the compression ratio in the Atkinson cycle, a mechanism with an inner piston that can provide such a motion control in a differential stroke engine is needed. In order to measure both of the inner piston motion and the cylinder pressure of a differential stroke engine, this thesis is focused on building up a measurement system to inspect the design of a mechanism that achieves the motion needed in a differential stroke engine. To obtain the inner piston motion in a differential stroke engine operated at a speed, the measurement system is equipped with a laser sensor and an angular encoder to identify the difference between the theoretical and experimental results. Also, to inspect the cylinder pressure, a pressure transducer is employed with the test bed so that both of the P-T diagram and P-V diagram can be calculated by using the measured result. To demonstrate the effectiveness of the developed test bed, real experiments are performed. The developed measurement system has proved to be a useful tool during the developing processes of design and manufacture of differential stroke engines.