Abstract: Chaos characteristics of the combustion process in an electronic controlled injection,spark ignition natural gas engine are studied using non-linear data analysis technology and chaos theory.The analysis of time series of multi-cycle in-cylinder pressure and reconstructed attractor in phase-space indicates that as spark advanced angle (SAA) increases,indicated mean effective pressure (IMEP) increases,coefficient of cyclic variation (COV_IMEP) decreases.The attractor of the combustion system in phase space has a twist,intercrossed and folded structure,with nonperiodic order.The correlation dimension (CD) of the reconstructed attractor is a fractional less than 2,and largest Lyapunov exponent (LLE) is positive,which means that the combustion system of natural gas engine is a low-dimensional chaos system.When SAA increases from 30°CA to 45°CA,IMEP increases from 0.798 MPa to 0.854 MPa,COV_IMEP decreases from 4.4% to 1.2%,CD decreases from 1.76 to1.59,and LLE decreases from 0.1 to 0.04;therefore both of the complexity of the combustion system and sensitivity of the system to initial conditions become smaller.