Background: Noise as a harmful physical agent for factory workers is one of the most important causes of neurosensory hearing loss. In this study, workers of noise-polluted factories were assessed by audiometric equipment to determine their frequency and intensity of hearing loss (HL). Materials and Methods: In this descriptive study 2004 workers from five factories in Zanjan and suburbs (including Iran-transfo, Pars-switch, Foolad-agfa, Zanjan-choob & Shir-e-pasturizeh) were selected through total sampling and assessed for audiometry. Cases with history of hearing impairment and conductive hearing loss were excluded from the study. Hearing of at least 30 dB in the band of 4KH was considered as the threshold of H.L. The results were classified and presented through descriptive statistics. Results: Out of 2004 cases 1395 (69.6%) were in normal range of hearing (threshold), 1 with previous history of H.L, 58 (2.9%) conductive H.L. and 7 with mixed H.L. 543 (27%) had isolated neurosensory H.L. of whom 311 cases had bilateral involvement and 124 and 108 cases had unilateral involvement of left and right ears, respectively. After excluding the cases with neurosensory H.L. probably related to old age, the final rate of frequency of noised-induced H.L. was obtained as 22.5%. The intensity of H.L. in each one of the ears was as follows: 53.6% mild H.L., 27.3% moderate H.L., 14.9% moderate to severe H.L, 2.9% severe H.L. and 1.3% of cases profound deafness. Conclusion: Similar internal and foreign studies have shown different results which are mainly due to various levels of noise and different cultural conditions of noise control in different factories. The results of this study indicate the necessity of care for utilizing protective instruments against noise in factories. More research concerning the change in temporary or permanent hearing threshold is recommended.