An optically accessible constant volume chamber was adopted to investigate the combustion and soot emission characteristics of soybean biodiesel under 800, 900, 1000, and 1200 K initial ambient temperatures. Results showed that with the decrease of initial ambient temperature ignition delay prolonged and heat release rate transited from mixing controlled combustion to pre-mixed dominate combustion mode. As the initial temperature decreased, natural flame luminosity gradually decayed, which indicated lower soot emission at lower initial ambient temperature.