This paper presents an experimental and theoretical study on limit conditions for smoldering combustion of biomass fuel. A fuel rod was burned in an oxidizer flow in a Tsuji-Yamaoka type, counterflow configuration, and the spread rate and temperature distribution were measured. The spread rate and maximum temperature increased with an increase in oxygen mass fraction in the oxidizer flow. Smoldering spread was not possible when the oxygen mass fraction was below a lower critical value, whereas a smoldering-to-flaming transition (SFT) occurred when the mass fraction was above an upper critical value. A one-dimensional model was then developed based on experimental observations to describe steady smoldering spread. The limit conditions were predicted by assessing the existence of solution. It was found that extinction occurs due to finiteness of char-oxidation rate, while the SFT limit can be predicted by considering gas-phase combustion of pyrolysis products.