Recent earthquakes in the near field have caused irreparable damages to the structures. The response spectrum of the near filed ground motions shows large spectral velocity and displacement in the low frequency range. Also the spectral accelerations of near field ground motion are amplified in the high frequency range for the rock site, and in the low frequency range for the soil site. The seismic behavior of the diaphragms against these ground motions have ambiguities and complexities. On the other hand, with modern architecture drawings, long span slabs that reduce architectural constraints are highly regarded. Some advantage of long span slabs are: creating large spans and consoles, possibility of creation of large openings in the slab, reducing the number of columns and reducing the weight. In this paper the seismic behavior of RC buildings with simple and waffle slab is investigated. Length of spans in RC buildings have different value of 5, 7, 10, 14 and 20 meters. The seismic behavior of these buildings under near field and far field ground motion are studied. The results shows that in buildings with similar span lengths, waffle slabs structure have less drifts. The application of waffle slab instead of simple slab in long span structures reduces the weight of building and the ratio of base shear to weight. But waffle slab in short spans structures don’t recommended because of increasing the weight. It is also observed that the behavior of structures under far field records is better than near field ones.