In this investigation, bacterial agents involved in pneumonia of slaughtered cross-bred cows in Tabriz abattoir were studied due to importance of bacterial pneumonia among Tabriz cattles. For doing so, we referred to Tabriz slaughter house every season and fifty healthy and fifty unhealthy lungs were sampled and transferred to the microbiology laboratory of Tabriz veterinary faculty, they were cultured then colonies were studied and finally the family and species of bacteria were recognized by using differential culture media. The separated bacteria from pneumonic lungs in four seasons involved 20 Streptococcus pneumoniae, 23 Escherichia coli, 18 Staphylococcus Aureus, 1 Staphylococcus epidermidis, 9 Corynebacterium pyogenes, 11 Pseudomonas aeruginosa, 48 Pasteurella haemolytica, 8 Erysipelothrix inscidiosa, 23 Rodococcus equi, 4 Nocardia farcinica, 2 Moraxella bovis, 1 Bordetella bronchiseptica, 2 Brucella bovis and 2 Haemophilus influenza. In four unhealthy lungs in fall, Pasteurella heamolytica with Bordetella bronchiseptica and from three lesioned lungs in fall, Brucella bovis with Haemophilus influenza and in three lesioned lungs in winter, Pasteurella heamolytica with Brodetella bronchiseptica were separated. The Bacteria isolated from healthy lungs consisted of 3 Staphylococcus epidermidis, 2 Pasteurella multocida, 6 Escherichia coli and 1 Nocardia farcinica. From 200 healthy lungs which were studied, 2 cases in spring, 4 cases in summer, 4 cases in fall and 2 cases in winter were isolated. It should be noted that, no bacteria was removed from primary culture of healthy and lesioned lungs in anaerobic conditions and mycoplasma culture.