Thermal decomposition of cellulose was carried out in a tubular reactor under flowing argon gas at 573, 673 and 773 K. The loss of carbon, hydrogen and oxygen was observed, and enhanced with increase in temperature continuously. The persistent free radicals (PFRs) were not observed in the sample decomposed at the lowest temperature of 573 K, and observed in the samples decomposed at higher temperatures of 673 and 773 K. This incontinuous trend was discussed from viewpoints of radical formation and its stabilization during the thermal decomposition. An empirical correlation between PFR density and residual carbon amount was proposed. © 2020