Effects of variations in natural gas composition on the autoignition of natural gas under direct-injection (DI) diesel engine conditions were studied experimentally in a constant-volume combustion vessel and computationally using a chemical kinetic model. Four fuel blends were investigated: pure methane, a capacity-weighted mean natural gas, a high-ethane-content natural gas, and a natural gas with added propane typical of peak shaving conditions. Experimentally measured ignition delays were longest for pure methane and became progressively shorter as ethane and propane concentrations increased. At conditions characteristic of a DI compression ignition natural gas engine at Top Dead Center (CR = 23 : 1, p = 6.8 MPa, T = 1150 K), measured ignition delays for the four fuels varied from 1.8 ms for the peak shaving and high ethane gases to 2.7 ms for pure methane. A computational model, incorporating detailed chemical kinetics of oxidation of methane, ethane, propane and other small hydrocarbons was used to predict the influences of fuel composition on ignition, focusing on the four fuel types considered in the experimental study. Numerically predicted variations in ignition delay as a function of natural gas composition agreed with these measurements. The model results are used to interpret the kinetic factors responsible for the observations.