To reduce the energy consumption of wood drying and optimize drying process for oak （ Quercus mongolica） ,the authors studied the effect of specimen thickness,initial moisture content and ambient temperature on preheating time. The results indicated that preheating time increased with specimen thickness in a non-linear relationship. Preheating time decreased with increasing ambient temperature. The initial moisture of the sample was found to have no obvious impact on preheating time. They suggested that the suitable preheat ambient temperature is around 70 ℃; preheating time could be appropriately prolonged when the initial moisture content of oak lumber is below fibre saturation point.