The stem radius growth (SRG) of six samples of Picea meyeri and five samples of Larix principis-rupprechtii nearby the tree-line of Luya Mountain in North China was measured by the dendrometer in the summer phase from July 15 to August 7 and the autumn phase from September 5 to October 9, 2007, with the environmental factors measured simultaneously. In the summer phase, there was no significant difference in the sensitivity of SRG to the environmental factors fluctuation between the two species; but in the relatively cold and dry autumn phase, the SRG of P. meyeri was more sensitive to the fluctuation of environmental factors. The accumulated SRG of the two species increased linearly in summer phase, but decreased first and kept stable then in autumn phase, with the growth increment of P. meyeri fluctuated more than that of L. principis-rupprechtii. The SRG of the two species correlated positively with hydrological factors and negatively with thermal factors, and the SRG of P. meyeri was strongly affected by air humidity and temperature, while that of L. principis-rupprechtii was mainly affected by soil moisture.