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Background and purpose: An increasing emergence of multidrug resistance among Klebsiella pneumoniae nosocomial isolates has limited the therapeutic options for treatment. The beta-lactamases are the major defense of gram-negative bacteria against antibiotics. The aim of this study was the detection of ESBLs and Amp-C enzymes and MOX, FOX, and ACT genes in K.pneumoniae strains isolated from hospitalized patients in Tehran hospitals during 2011-2013. Materials and methods: This study was conducted in 120 K. pneumoniae isolates from Imam Hossein, Taleghani and Mofid Children's hospitals in Tehran, Iran. Antibiotic susceptibility tests were performed using Kirby-Bauer disc diffusion and Broth Microdilution methods. The detection of Amp-C and ESBLs enzymes was carried out according to CLSI guidelines. The MOX, FOX, and ACT genes were detected by PCR and sequencing methods. Results: Among 120 K. pneumoniae strains, 24 (20%) and 68(56.8%) were Amp-C and ESBL positive, respectively. In this study colistin and tigecycline were found more active than other antibiotics. ACT, FOX, and MOX genes were detected in 16 (13%), 46 (38.33%) and 43(35.83%) of the isolates, respectively. Conclusion: The prevalence of antibiotic resistance genes detected in this study is of great concern and highlights the need for infection control measures including antibacterial management and identification of resistant isolates.