The effect of natural gas composition on ignition delay has been investigated numerically by using detailed and reduced chemical kinetic mechanisms. Three different blends of natural gas have been analyzed at pressures and temperatures that are typical of top dead center conditions in compression ignition engines. The predicted ignition delay shows a decrease with temperature in an Arrhenius manner and has a first order dependence on pressure. Similar trends have been observed by Naber et al.  in their experimental study of natural gas autoignition in a bomb. It is shown that two kinetic mechanisms (GRI-Mech 1.2 and reduced set DRM22) are best capable of predicting the ignition delay of natural gas under compression ignition conditions. The DRM22 mechanism has been chosen for further studies as t involves lower computational costs compared to the full GRI-Mech 1.2 mechanism. Parametric studies have been performed using this mechanism to investigate the effect of fuel composition, ambient density, equivalence ratio and compression ratio on ignition delay of natural gas. The computations reveal a strong dependence of ignition delay on natural gas composition, with pure methane having the highest delay, followed by blends with increasing percentages of higher hydrocarbons.