Smoldering combustion is an important form of combustion in wildfires and hazard reduction burning because it plays vital roles in pollutant emission, fire re-ignition, and ecological impact. Smoldering combustion can be classified as either radiation-aided or self-sustained, depending on the nature of the reactions. The latter is often considered a more hazardous type of smoldering combustion, because it can persist for a long period of time and can transition into flaming combustion. However, there is a lack of understanding of the differences between radiation-aided and self-sustained smoldering combustion processes, especially regarding characterization. The aim of this study is to investigate and quantify the differences between radiation-aided and self-sustained smoldering combustion in biomass. Experiments were conducted using an infrared heat lamp to heat pulverize fuel samples in a reactor. The external energy input and oxygen concentration were controlled in order to achieve radiation-aided and self-sustained smoldering combustion. Radiation-aided and self-sustained smoldering combustion were quantified based on temperature measurements in the reactor, the analyses of product gases, and the mass change of the testing samples. Under the current experimental conditions, self-sustained smoldering can only be initiated when the oxygen concentration is between 10% and 21%; only radiation-aided smoldering combustion can be initiated in oxygen concentrations under 7.5%; and no ignition occurs when the oxygen concentration is equal to or less than 5%. From the temperature measurements, there is a linear relationship between oxygen concentration and smoldering velocity.