With the increasing interest in well-developed fires, the burning behavior of furniture under flashover should be studied. Full-scale burning tests on the heat release rates of furniture under flashover with oxygen calorimetry will be reported in this article. Nine tests on sofas with and without the treatment of fire retardant, wood desks, and the fire source itself to start the burning were carried out. The heat release rate, oxygen concentration, thermal radiative heat fluxes at floor level, and air temperatures at some selected positions were measured. Response of furniture under small accidental fires and bigger pool fires to onset flashover will be discussed. Obviously, burning furniture under a flashover fire will give a much higher heat release rate. Sofas treated with fire retardant might burn even more vigorously. This point should be considered carefully in designing fire-safe furniture.