Leptospirosis is a world-widely spread zoonotis disease with numerous sources which is caused through the infection produced by the pathogenic spirochetes of the genus Leptospira. This research was conducted in order to determine the infection rate of the Leptospira epidemic in the cattle of Zanjan Province industrial slaughter house, southwest of Iran in 2011. In this study, a total of 135 random serum samples were collected from cows. All serum samples were serologically tested by microscopic agglutination test (MAT). The serum samples were tested for antibodies against eight live antigens of Leptospira interrogans serovars: Australis, Icterohammorrhagiae, Hardjo, Grippotyphosa, Canicola, Autumnalis, Pomona, and Sejroe.  The results of this study showed that 60 (%44.4) animals had a positive reaction against one or more serovars. The most prevalent Leptospira serovars was Hardjo and the least one belonged to serovar Pomona. The lowest dilution that each serum was considered positive was 1:200.  Based on the results of this study and by considering higher abundance of prevalence of leptospira infection and dominant titer of 1:200 reveal that leptospiral infection among the cattle of the Zanjan Province Slaughter House is explanatory of the Leptospira infection for which the required hygienic measures towards preventing the Leptospirosis disease ought to be taken.