It is very important to know the dynamic behavior of base slabs of buildings in earthquake prone areas. The slabs are generally assumed as a rigid diaphragm in structural analysis and design. But their response is considerably affected from ground condition. In this study, the dynamic characteristics of a scaled base slab are determined for different ground conditions. The dimensions of the base slab model are 150 cm long, 100 cm wide and 5 cm thick. This model is designed to be smaller than 20 times from an actual building base. The base slab model is tested for the ground conditions of sand, gravel and clay-silt mixture by ambient vibration test. The dynamic characteristics, such as natural frequencies, modal damping ratios, mode shapes, are identified from collected signals by Operational Modal Analysis method. The first five modes are taken into consideration to compare the effect of ground conditions on the dynamic characteristics. It is observed that the natural frequencies, mode shapes and modal damping ratios are considerably affected from ground conditions. The highest natural frequency is obtained for the gravel ground case. The lowest natural frequency is identified for the clay-silt mixture ground case. The mode shapes changed randomly in each case. But the main modal behaviors are vertical mode and bending modes. (C) 2014 Elsevier Ltd. All rights reserved.