The goal of this study is to investigate the use of compressed natural gas in a diesel engine compared to diesel fuel, according to exergetic and exergoeconomic analyzes. For this purpose, compressed natural gas (CNG) injector has been mounted to intake manifold and small, single cylinder diesel engine was converted to dual mode. Experiments have been carried out according to ISO 8178 E2 test cycle, at maximum power and four different load as 25%, 50%, 75%, and 100%. CNG ratios were chosen as 0%, 20%, 40%, and 60% percentages as energy content. Exergetic and exergoeconomic analyses were executed to results considering capital, operation-maintenance, fuel and environmental damage costs. According to results, diesel fuel has superiority than CNG fuels except full load. In these loads, with using CNG, exergetic efficiency and exergoeconomic factor reduces as up to 38.3% and 32.8%, and also exergy destruction percentage, lost exergy rate and specific cost of shaft work exergy unit increases as up to 10.8%, 74.8%, and 41.7%, respectively. Besides CNG fuels need to improve lower load conditions performance, CNG20 has the favorable exergetic results at full load. According to environment related cost results, remarkable cost of UHC affects negatively to CNG fuel results at 25% and 50% loads. Last of all, CNG20 is preferable at 75% load and all CNG fuels are preferable at full load conditions according to total cost flow rates against diesel.