Abstract: To examine the relationships between xylem anatomical characters of tree stem and sap flow stem CO2 efflux, we instrumented Betula platyphylla, Larix gmelinii and Flaxinus mandshurica to measure the stem CO2 efflux using infrared gas analysis (IRGA) method in situ, and simultaneously measured sap flow density and stem temperature, as well as the anatomical characters of the xylem vessels and tracheid of the three tree species using casting method. The results showed that sap flow density changes dynamically in the three species with the same trend. Significant differences of sap flow density among the three species were found. Mean sap flow density of birch was 38.01% and 65.80% higher than of ash in June and September, respectively, and for larch that was about 82.91% and 71.89% higher than ash. The difference in stem CO2 efflux among the trees was also significant. The figures of ash were 1.92 and 2.17 times of birch and 1.49 and 2.17 times of larch in June and September, respectively. There were differences in catheter molecular lumen length, width, side wall slope angle, type of the secondary duct wall thickening and pits between trees, and stepwise regression analysis showed that vessels (tracheids) lumen diameter was the main factor affecting sap flow density and stem CO2 efflux. With the increase of lumen diameter, sap flow density showed decreasing trend while stem CO2 efflux showed a contrary trend. Our results imply that sap flow density can influence the stem CO2 efflux to some extent; therefore, the indirect impact of conducting tissue structure on stem CO2 efflux could show up.