Abstract: Based on the fact that the half-sum linear combination of code and phase observations may remove the first-order ionospheric error,a mathematic model of the combined GPS and GLONASS precise point positioning(PPP) using UofC ionosphere-free combination is developed.The UofC PPP model estimates individually ambiguity parameters on two frequencies,which facilitate further obtaining integer solutions of ambiguities.The positioning results obtained from the UofC model and the traditional model in which code observations on two frequencies are used to form ionosphere-free combinations are compared using GPS/GLONASS observation data from IGS tracking stations.The statistical results indicate that the UofC model performs slightly better in terms of the horizontal position accuracy.But the difference about the three-dimensional positioning accuracy is insignificant.