Respiratory disease is the most common cause of sickness and mortality in the feedlot. The selection of an appropriate antibiotic and the evaluation of its success in the treatment of respiratory disease is an important consideration to the feedlot owner and his veterinarian. The aim of this study was to investigate penicillin (3 million IU) in treatment of pneumonia in calves. In this study, forty crossover calves of 3-6 months of age with an average weight of 60 to 100 kg with pneumonia were clinically examined. Understudied calves were distributed in two groups, in control group for treatment penicillin (3 million, Nasr pharmaceutical drugs Co.) was used, and in the other group the penicillin (3 million Erfan pharmaceutical drugs Co.) was used. During the treatment phase of patients with pneumonia in control and experimental group, sterile swabs was taken from the lower part of the URT of infected patients, and simultaneous with antibiotic therapy, the taken sterile swabs were undergoes bacteriological examination and types of bacteria was characterized. Statistical results indicated there is not any significant differences between two different penicillin in antibiotic susceptibility (p>0.05). Researchers constantly have been tried new producing drugs but have not achieved the desired product due to the wonders of life and biology and although these products are good in times. It seems that penicillin procaine injection powder (3 million IU) according to our study results may be effective in the treatment of pneumonia in calves with the effective range.