Abstract: Plant temperature is an important parameter for estimating energy balance and vegetation respiration of forest ecosystem. To examine spatial variation in diurnal courses of stem temperatures (T_s) and its influencing factors, we measured the T_s with copper-constantan thermocouples at different depths, heights and azimuths within the stems of two broadleaved tree species with contrasting bark and wood properties, Betula platyphylla and Fraxinus mandshurica. The results showed that the monthly mean diurnal courses of the T_s largely followed that of air temperature with a ‘sinusoi-dal’ pattern, but the T_s lagged behind the air temperature by 0 h at the stem surface to 4 h at 6 cm depth. The daily maximal values and ranges of the diurnal course of T_s decreased gradually with increasing measuring depth across the stem and decreasing measuring height along the stem. The circumferential variation in T_s was marginal, with slightly higher daily maximal values in the south and west directions during the daytime of the dormant season. Differences in thermal properties (i.e., specific heat capacity and thermal conductivity) of both bark and wood tissue between the two species contributed to the inter-specific variations in the radial variation in T_s through influencing the heat exchange between the stem surface and ambient air as well as heat diffusion within the stem. The higher reflectance of the bark of B. platyphylla decreased the influence of solar radiation on T_s. The stepwise regression showed that the diurnal courses of T_s could be well predicted by the environmental factors (R² > 0.85) with an order of influence ranking as air temperature > water vapor pressure > net radiation > wind speed. It is necessary to take the radial, vertical and inter-specific varia-tions in T_s into account when estimating biomass heat storage and stem CO₂ efflux.