Abstract: We investigated CO2 concentration in the Beijing Olympic Forest Park between 2008 and 2009. Ten sampling points, including nine inside the park and one outside the park as control, were chosen and CO2 concentration was measured in clear days of April, July, November of 2008 and January 2009, respectively. The results indicated that: 1) the diurnal variation of CO2 concentration in the park was significant, presenting generally a picture of decreasing from 08:00-11:00, then fluctuating at noon around a low concentration point (approx. 310 μmol/mol), reaching the minimum concentration (approx. 300 μmol/mol), and increasing significantly afterwards; 2) variation characteristics of CO2 concentration in the non-growing season was similar to that in the growing season, except for the daily average concentration (413 μmol/mol, 328 μmol/mol in the growing season) and the time of the minimum concentration (13:00 pm, 14:00 pm in growing season); 3) it had the lowest concentration (328 μmol/mol) in the summer and the highest in the autumn and winter (413 μmol/mol), but no significant differences existed between the spring (410 μmol/mol) and the winter; 4) the CO2 concentration in growing season followed a gradient change, decreasing gradually from near the border of the park (360 μmol/mol) to the buffer area (336 μmol/mol) and to the center of the park (332 μmol/mol). The results indicate that the high vegetation coverage and abundant plant community structures decreased CO2 concentration in the park area, and the respiration of vegetation and soil could also have some effects.