Background and Objective: According to previous studies shift work could desynchronize the natural circadian rhythm of the body. Although some of the internal physiologic processes become active for adaptation of the body with this desynchronization. One of these physiologic processes is endocrine system and melatonin release. This hormone is one of the most important variants which represent the circadian rhythm in human. Since the pattern of secretion of this hormone in first ours of morning and during the day in Iranian workers in particular in shift workers is unknown, therefore we aimed to determine the 24 hours profile of melatonin in shift work and permanent day shift nurses. Materials and Methods: This experimental study was carried out on forty four female nurses of the Shiraz university hospital, during 2006-2008. Thirty four people in study group had a cyclic shift work and 10 persons in control group had permanent day shift work. The serum samples with 3 hours intervals during 24 hours were taken from each person. The plasma concentration of melatonin was measured by ELISA. Our study was carried out under realistic conditions. The data were analyzed using one –way ANOVA. Results: The age range was between 22 to 50 years with a mean work history of 5.5 years. The highest and the lowest melatonin levels was found in shift work nurses at 04:00 pm (14.91 pg/mL), and 04:00 am (131.49 pg/mL). These values for permanent day work nurses in the same times was 1.02 pg/mL and 177.40 pg/mL respectively. There was a significant difference between circadian melatonin profile at different time points (P= 0.000). Conclusion: The results of this study revealed that night work induces a consistent change in melatonin circadian profile with a progressive reduction at early morning (04:00 am) and awaking time (07:00 am). These changes will also disturb sleep cycle and level of consciousness during the night and activities during the day.