Abstract: In case of triple-frequency signals, there are many combination observations with longer wavelength, smaller noise, smaller ionosphere effect, which may improve the precision of cycle-slip detection and repair. The threshold for cycle-slip detection is deduced, and the success probability of cycle-slip determination is proposed. Criterions and methods for selecting code-phase combination are presented from the viewpoint of enhancing the success probability of cycle-slip determination. At last, experiments are carried out to test the performance of the method using GPS triple-frequency observations. The result shows that optimal code-phase combinations selected by proposed criterions can detect and repair all cycle-slips of triple-frequency un-differenced observations, when ionosphere error between-epoch can be ignored in case of high data rate.