Vertical distributions of cloud condensation nuclei (CCN) in a cloudless sky were observed by the DMT (Droplet Measurement Technologies) CCN counter mounted on an AN26 airplane in the western Guanzhong area of Shaanxi Province on 13 October 2009. The results of observation show that the ground average concentration of CCN was more than 10000 cm-3 (supersaturation was 02%) under a cloudless sky condition, and the ground CCN concentration for foggy days on 7 October 2009 was significantly higher. The CCN concentration tends to decrease with height. CCN concentration was high below 1500 m, with the mean CCN concentration being 8827 /cm3, 4439 /cm3 , 456 /cm3, 504/ cm3 (supersaturation equals 04%) at the levels of 1000 m, 1550 m, 4100 m, 5500 m, respectively.