To realize large-percentage biomass co-firing with coal in existing coal-fired boilers, the combustion behavior of biomass is expected to be similar or comparable to that of coal. When co-firing with coal, biomass is not necessarily to be ground as fine as the dedicated coal particles, because of its higher reactivity. With the objective of achieving promising performance of co-firing with dedicated coal particles, the determination of suitable biomass particle size becomes important. The paper investigates experimentally the ignition behavior of three biomass materials in a down-fire reactor associated with thermogravimetric analysis (TGA). TGA results showed that the devolatilization process is accelerated by the presence of oxygen, but failed to identify the impacts of particle size on the ignition behavior of biomass. However, the ignition testing results based on the down-fire reactor clearly show that the ignition delay time of a large biomass particle is longer than that of a smaller one. In addition, when injected into the furnace, the softwood particles require a longer residence time to be ignited than the straw particles of the same size, which agrees well with their reactivity analysis in TGA. Moreover, the ignition test results suggested that the ignition mechanism of biomass could be alternated from homogeneous ignition to heterogeneous ignition when the furnace temperature is increasing; at sufficiently high furnace temperatures, the ignition predictably occurs at the particle surface without requiring the start of devolatilization. The results quantitatively demonstrate the effects of particle size on the ignition delay time of biomass, which, together with the transport phenomena and surrounding atmosphere, can contribute to control the biomass combustion profile and co-firing performance.