Abstract: The offaxis integrated cavity output spectroscopy technique was used to measure air CO₂ concentration and stable carbon isotope ratio (δ¹³C) above (11 m) and at the bottom (6 m) of canopy of a Quercus variabilis plantation in a low hilly area of North China. The variations of CO₂ concentration and δ¹³C value in Q. variabilis plantation canopy and the influencing factors were analyzed at hourly timescale. The results showed that diurnal variation in the CO₂ concentration had a trend, while there was no obvious similar tendency in the diurnal change of δ¹³C value. Daytime atmosphere stability frequency during the experiment time was 70.2%. With the combined effects of photosynthesis and turbulent in the canopy, CO₂ concentration at the bottom of canopy was 1.70 μmol·mol^-1 higher than that above the canopy, while the δ¹³C value was 0.81‰ lower than that above the canopy. Atmosphere stability frequency was 76.2% at night. The CO₂ from leaf was not easy to move because of weak turbulent. Thus, CO₂ concentration at the bottom of canopy was 1.24 μmol·mol^-1 higher than that above canopy, while the δ¹³C value was 0.58‰ lower than that above canopy. The difference of CO₂ concentration between above and at the bottom of the canopy was strongly correlated with their δ¹³C difference both in daytime and at nighttime. Stepwise regression analysis indicated that solar radiation and relative humidity in daytime were the main environmental factors causing CO₂ concentration and δ¹³C difference between above and at the bottom of the canopy, whereas at nighttime temperature was a key environmental factor influencing δ¹³C value. The above environmental factors strongly influenced CO₂ concentration and δ¹³C value in air above and at the bottom of Q. variabilis plantation canopy by increasing or decreasing photosynthesis and respiration.