This study aims to investigate heavy metal accumulation in water-sediment, plankton, and macrophyte samples of Lake Iznik, one of the important lakes of Turkey. In 2016, water-sediment and plankton samples were taken from 19 and 10 stations respectively. In addition, heavy metal analyzes were performed on samples taken from Potamogeton pectinatus, Najas marina, Myriophyllum spicatum, Typha latifolia, Schoenoplectus litoralis, Phragmites australis, which are the most abundant macrophytes in the lake. The data of water and sediment samples were spatially analyzed in GIS and mapped with ArcGIS. In the water samples of Lake Iznik, arsenic (As), mercury (Hg) and in sediment samples, arsenic (As), chromium (Cr), copper (Cu) and nickel (Ni) were detected at remarkable levels. The results revealed that Iznik Lake was under pressure in terms of heavy metals and their effects should be reduced.