It is important to understand the formation of hydrogen cyanide (HCN) for minimizing the emissions of NOx during biomass combustion. In the present study, glycine was used as the model compound of biomass N. The influences of reducing sugars (glucose and fructose) on the formation of HCN from glycine pyrolysis were investigated by analyzing the yields of HCN, 2,5-diketopiperazine (DKP), and N-heterocyclic compounds. The results indicated that the HCN yield from glycine pyrolysis decreased and the nitrogen distribution changed in the presence of glucose/fructose. Of the thermal reactions, glycine might be more likely to react with reducing sugars to form a large number of N-heterocyclic compounds rather than polymerize to form DKP. The decrease of the HCN yield from co-pyrolysis of glycine and reducing sugars is related to the reduction of DKP and the increase of N-heterocyclic compounds. Furthermore, analysis of label incorporation into HCN formation at different molar ratios of glycine/reducing sugar indicated that the contribution of reducing sugar to HCN formation might be more considerable with the increasing content of reducing sugar.