Aircraft survivability is an important factor in military aircraft mission, and stealth technology is required to reduce the possibility of detection in order to improve aircraft survivability. Reducing the high-level infrared signal radiated by high-temperature engine surface and exhaust gases from aircraft propulsion engine is a key technology for improving aircraft survivability. Currently most of unmanned aircraft vehicle uses turbofan propulsion system with various bypass ratios. Therefore, it is necessary to study the characteristics of the infrared signal radiated from the exhaust gas of the turbofan engine having various bypass ratios. In order to understand the characteristics of the infrared signal radiated from the turbofan engine, it is necessary to carry out research using a real engine. However, it is very difficult to use real turbofan engine because the turbofan engine is very expensive and require specialized test facility. In this study, micro turbojet was modified to simulate the real turbofan engine behaviour. The jet flow of micro turbojet engine simulates core flow and bypass flow is simulated by external high pressure air source. The effects of bypass ratios of 0.5, 1.0 and 1.4 are examined. The experimental results show that infrared signal decrease as the bypass ratio increases. And also gas temperature decrease with bypass ratios. Especially, infrared signal and exhaust gas temperature is largely reduced near the nozzle exit as the bypass ratio increases. Schlieren visualization of the exhaust gas plume was executed to confirm the plume temperature distribution. Comparing the temperature distribution and Schlieren images, the structure and behaviour of turbofan plume with various bypass ratio were understood.