In the ground tests of hypersonic scramjet, the high-enthalpy airstream produced by burning hydrocarbon fuels often contains contaminants H2O and CO2. The contaminants may cause the ignition characteristics of the fuel become discrepant between ground tests and real flights. In order to properly assess the influence of the contaminants on ignition characteristics of hydrocarbon fuels, the effects of H2O and CO2 on ignition characteristics of ethylene were investigated in a preheated shock tube. Based on the ignition delay times of ethylene in clean gas, comparative experiments with clean and vitiated gases were conducted. At pressure of 0.2 MPa, equivalence ratios of 1 and 0.5, ignition delay times of ethylene were measured for four cases: 1) clean gas, 2) gas vitiated with H2O mole fraction of 7.5%, 15% and 25% by mole, 3) gas vitiated with CO2 mole fraction of 10% by mole, and 4) gas vitiated with H2O mole fraction of 25% and CO2 mole fraction of 10% by mole. The experimental results show that water vapor and carbon dioxide have almost no effect at equivalence ratio of 0.5, but an inhibiting effect appears at equivalence ratio of 1. An obvious inhibiting effect appears over a wide temperature range when water vapor and carbon dioxide exist together. The results were also discussed preliminary by considering both the combustion reaction mechanism and the thermophysic properties of the test mixtures.