Fast pyrolysis, with high heating rates and short retention time, was studied extensively in past decades due to high liquid yields. Although many fast pyrolysis processes have been developed and tested, few were reported to be successful on commercial scale due to problems including poor tar quality and system plugging. It is, therefore, important to exam the reactivity of the fast pyrolysis tars particularly at temperatures they may experience upon releasing from the coal surface. Four coals of different ranks were pyrolyzed to study the reactivity of the tars. It is found that the tars contain high concentrations of radicals and are highly reactive at temperatures higher than 623 K to generate more radicals and form coke. The coke formed in the tars contributes most of the radicals in the tar samples. The coking behavior can be expressed by the second order kinetics with activation energies of around 200 kJ/mol. The coal tar's reactivity is significantly less than that of biomass tars obtained from the same pyrolysis reactor under the same conditions.