The climatology of ionospheric slab thickness is analysed using dual-frequency GPS and COSMIC measurements in China during the solar minimum. It is found that a pronounced peak of slab thickness occurs during the post-midnight period. A large diurnal ratio exists at the EIA, and a large night-to-day ratio occurs near the equatorial latitudes and mid- to high latitudes. The behaviours of the slab thickness and the mF2 are well correlated at the middle latitudes and during the daytime.