Abstarct: Tirgan carbonate formation in the structural zone of Kopet-dagh contains significant groundwater resources. In this region, Tirgan Formation is not a homogeneous formation but consists of five rock units with different permeability. This study was performed to investigate hydrostratigraphy and separation of permeable laxyers in Tirgan Formation. A stratigraphic section was selected in the northeast of Bojnourd, 90 thin sections of rock were used to analyze the sedimentary environment. Geophysical well logs, 270 joints information at three stations, and step-by-step pumping tests were used. Tirgan Formation consists of limestone, marl and marly- limestone rock units. The results obtained from this study show that the purity of upper limestone rock units has increased. The hydrogeological characteristics of Tirgan Formation are different in each horizon due to the role of stratigraphic parameters of the formation. According to the analyzed pumping data, the maximum and minimum values of transmissivity (T) are estimated at 1040 and 130 m^2/d respectively. Also, the maximum and minimum hydraulic conductivity values are calculated to be about 15 and 1.2 m/d, respectively. The main reason for the large difference in hydrogeological parameters in this region is related to the effect of stratigraphic parameters. Using the drawing method presented by Karami (Karami 2002), the” Karst conduit index”, which is a quantitative exxpression of the dominant flow system, is introduced in this research. This index is located between the boundaries of +1 and -1, which values between zero and +1 express the predominance of conduit flow system and values between zero and -1 express the predominance of diffuse flow system. In this area the results obtained from the evaluation of karst conduit index of pumping wells indicate that diffuse flow system is predominant. In this study, for the first time, a new index for quantitative evaluation of stratigraphic parameters affecting the hydraulic properties of karst is introduced, which is called “stratigraphic index” (SI). This index quantitatively calculates the effect of important stratigraphic properties such as thickness, laxyering and slope of the laxyer on the hydraulic properties of the karst. Evaluations indicate that the degree of heterogeneity, flow dimension, karst conduit index, well discharge, specific drawdown and transmissivity show significant relationships with stratigraphic index. Finally, it can be mentioned that the initial evaluation of stratigraphic parameters and the application of criteria such as stratigraphic index can help to more accurately locate well drilling in karst aquifers.