Plant transpiration conductance variables, including canopy conductance(G_c), canopy stomatal conductance(G_s) and leaf stomatal conductance(g_s), were able to characterize quantitatively the vapor transmission process between plant and atmosphere in the soil-plant-atmosphere continuum (SPAC) and to reflect the water regulation ability of plants. It was very important to discriminate the similarities and differences between the three factors mentioned above in reflecting the process of plant water use and the correlation among them for figuring out mechanism of plant water use. Based on the continuous observation of apple trees sap flow velocity (J_s) and environmental factors during the growing season on the Loess Plateau, G_c, G_s and decoupling coefficient (Ω) were calculated and compared with the g_s measured continuously for a few sunny days in field, and the relationships between G_c, G_s and g_s and the similarities and differences of them in estimating canopy transpiration rate were analyzed. Results showed that diurnal courses of G_s and g_s showed a "unimodal" curve while that of G_c showed a "bimodal" curve that was increase followed by decrease and recovered slightly in the afternoon. The linear relationship between G_s and g_s was close (R²=0.80) while that of G_c and g_s was weak (R²=0.02). G_c or G_s varied with vapor pressure deficit (VPD) were clear with the upper boundary function of decreasing logarithmic function while the relationship between means of that and VPD could be fitted well by Log-Normal function of first increase and then decrease (R²>0.95). The means of G_c and G_s varied with VPD showed turn points at 1.33 and 1.16 kPa, respectively. In daytime, the response process of G_s to VPD was consistent with that of g_s to VPDL (vapor pressure deficit calculated based on leaf surface temperature), and the consistency was higher than that of G_c to VPD. In the whole growing season (from April to October), the average Ω was 0.12. G_c linear correlated with G_s and the fitted line slope increased gradually with the decrease of Ω, which meant that G_c was close to G_s gradually. The results of research showed that canopy transpiration conductance of an individual or stand can be estimated accurately based on the sap flow in the northern China. Compared with the diurnal course of g_s measured in field, G_s was more consistent with the g_s than G_c. G_s was a more appropriate variable to reflect the response of apple trees water use process to atmospheric driving.