Granier-type thermal dissipation probes were applied to continuously measure the stem sap flow dynamics in oak (Quercus liaotungensis) and three other tree species (Armeniaca sibirica, Acer stenolobum, and Euonymus bungeanus) representing the dominant and companion species, respectively, during the growing season from May to October, 2011, in a natural secondary forest near Yan'an of Shaanxi Province in the loess hilly region. Solar radiation (Rs), air temperature, and relative air humidity were monitored simultaneously. The diurnal dynamics of sap flux densities and their responses to environmental factors for the four tree species (Q. liaotungensis, A. sibirica, A. stenolobum, and E. bungeanus) were analyzed. The results showed that patterns of diurnal variation of sap flux densities in the four species were similar, with a single-or double-peak curve, generally reflecting the changes in environmental factors (e.g., Rs). However, there were some differences among the four tree species. The starting time of sap flow in Q. liaotungensis was usually around 05:00, more than 1 hour earlier than the other three species. Sap flux densities in Q. liaotungensis reached daily peaks earlier than others, usually around 09:00, whereas A. stenolobum, E. bungeanus, and A. sibirica reached their daily peaks at 10:00, 11:00, and 13:00, respectively. The companion species A. stenolobum and E. bungeanus showed a relatively narrow peak in their diurnal patterns of sap flux densities, suggesting a limitation of radiation conditions within the community. In addition, soil moisture conditions showed a more obvious effect on companion tree species than on the dominant species. Correlation analysis indicated that sap flux densities were significantly correlated with both solar radiation and vapor pressure deficit during the monitoring period. The difference in sap flux densities between dominant and companion trees may be attributed to both properties of the tree species and redistribution of radiation by canopies.