A brown coal(Yallourn) was heat-treated in a nitrogen atmosphere, and the original and heat-treated coals were subjected to flash pyrolysis by using Curie-point and entrained-flow pyrolyzers. The yield of CO2, the most abundant product in the heat treatment stage, was varied by the heat treatment temperature in the range of 0.4-11 wt % and was employed as a measure for heat treatment severity. The heat treatment gave both positive and negative effects on the pyrolysis reactivity of coal. In the low-severity heat treatment regime where the CO2 yield was less than 5 wt %, the tar yield increased and the char yield decreased with increasing severity. The CO and H2O evolution was suppressed in the succeeding pyrolysis. This means that the decomposition of phenolic-OH groups leading to cross-linking was inhibited. The pyrolysis was promoted by the low-severity heat treatment, which relaxed hydrogen bonds. The THF swelling ratio was drastically lowered, due to the decrease in the content of carboxyl groups as interaction sites with THF, not due to the increase in cross-link density. On the other hand, the high-severity heat treatment with the CO2 yield higher than 5 wt % decreased both. tar and total volatile matters with increasing severity. The decrease in the CO yield by the high-severity heat treatment was attributed to the loss of -OH groups as a major source of CO during the heat treatment.