In order to characterize the trophic state of Iran's coastal waters, three coastal water quality indices, i.e. OECD (Organization for Economic Corporation and Development) index, NCS (Nutrient Concentration Scale) index and TRIX (Trophic Index), have been applied to existing water quality data. The study area was located in Hormozgan coastal waters, Persian Gulf. Chemical and biological parameters (PO4, NO3, NO2, NH 3, DO and Chl a), collected during three cruises in 1993 and 1994 were compared with those of Adriatic and Aegean seas and then the above mentioned indices were applied to them. Differences between the trophic states obtained from the above-mentioned indices showed that these indices have some restrictions and should be modified to be used in this area. Therefore, a new trophic index for Hormozgan coastal area was developed by modification of TRIX index. It is believed that this trophic index makes comparison between different trophic states more accurately.