Abstract: The slab thickness is an important ionospheric parameter and is defined as the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2). The variation of ionospheric slab thickness is analyzed by using GPS and ionosonde observations from August 2000 to April 2006 at Qingdao (120.3^oE, 36.0^o N). An enhanced peak of slab thickness occurs around 04-06LT in different season and solar activity. The slab thickness changes mainly between 200 and 500 kilometers. The slab thickness is greater for day-time in summer and for night-time in winter. The slab thickness of day-time reduces with the degradation of solar activity at same season, but the one of night-time changes less between high and low solar activities. Moreover, analyzes the distribution of the relative deviation of slab thickness and finds the perturbation of slab thickness happens easier in night-time than day-time.