This study was performed to evaluate the mildew resistance of heat-treated lumber. Dinitrosalicylic acid was used to estimate the sugar content in heat-treated birch and Mongolian pine samples. The results showed that heat-treated lumber was more susceptible to mold growth than their untreated control samples. Furthermore,the outside layers of the heat-treated samples were more prone to mold growth than the inside layers. No mold growth was observed on the surfaces coated with varnish. The sugar content of heat-treated samples increased with higher treatment temperature and the sugar content of the outside layers was usually higher than the core.