Abstarct: If equivalent statical method is used to analys a structure. baxse level will be one of the effective parameters to determine seismic forces on structure. in structures with same level foundation and without any intraction among structure walls and soil, it is obviously that baxselevel starts from foundation top surface, but civil engineers usually encounter buildings that there is an underground room in a part of structure. Therefore Foundation is built two different baxseline. because of that there is no suggestion realated to above problem in order to specify position of baxseline level at seismic regulations specialy in iran regulations 2800. designer engineer face to this question that realy to perform equivalent statical analysis which baxseline should be considered as basic baxseline. In this study tried to answer this question. To do this study, structural models with foundation and terrestrial environment around it, is modeled in abaqus software and according to soil type was set under a artificial acceleration meter comparison with regulation 2800 spectrum. in this study, two kinds of soils (I) & (IV) was applied that introduced in regulation. the results show that the baxse line location is severly under the in fluence of soil type and is also depended on the number of input columns to underground room and the number of structure floors.