Background and purpose: Hospitalized patients experience at least one hospital-acquired infection. Antibiotics are usual drugs used in such conditions that impose high cost burdens on patients and community. Moreover, they cause bacterial resistances. The purpose of this study was to evaluate the usage of antibiotics in intensive care unit in a teaching hospital. Materials and methods: A cross sectional study was conducted within six months in 101 critically ill patients who were on antibiotics in intensive care unit in Sari Bouali Sina Hospital. The patients’ and antibiotics’ information were recorded and then analyzed. Results: Among the samples 23.8% were not sent for bacteria cultures. In 58.4% there were negative cultures. Bacteria were detected in 41.6% and the comments were E. coli 24.4%, Klebsiella 17.1%, and Acinetobacter 9.8%. The most commonly used first-line antibiotics in hospital were ceftriaxone (72.3%) and meropenem (10.9%), while in second-line treatment meropenem was given more often. Conclusion: Suitable sampling, identifying the bacterial species, and selecting an appropriate antibiotic are of great importance in successful treatment of infections. Irrational use of antibiotics could lead to false negative results, and thereby bacterial resistances.