At a given drying condition,the water diffusion rate is usually determined by the size of lumber.In this study,the effect of the dimensions of poplar lumber specimens on the drying speed during vacuum drying was investigated.Specimens with different sizes and length-width ratios were divided into edge-sealed and unsealed（control） groups.Drying conditions under a constant vacuum of 0.02 MPa and temperatures of 45,65 and 85 ℃ were selected.The results showed that the critical dimension for Poplar increased when the drying temperature increased and decreased when the ratio of side area to volume decreased.