Display omitted •Contamination of MPs in Bohai Bay coast was at moderate level.•Total number of MPs in the sediments of Bohai Sea was 534 trillion.•Densities of some PE and PP particles were higher than that of seawater.•Density increasing rates were 7.4%–12.6% and 12.1%–17.5% for PE and PP, respectively.•Density increasing was caused by the co-contribution of biofouling and minerals. Size, shape and color are the basic parameters of environmental microplastic (MPs). However, the density of microplastics which is an important parameter to influence the fate of MPs in the environment has not been measured. Meanwhile, the MPs characteristics from coastal sediments of Bohai Bay remain unclear. In this study, the concentrations, distribution and composition (shapes and sizes) of MPs were investigated. The average concentration of MPs in the coastal sediments of Bohai Bay was 192.3 ± 138.9 items/kg dw, which was at a moderate pollution level. The MPs with fiber shape (54.5%), small size (52.0%) and low density (PE and PP) were dominant. In addition, flotation experiment showed that most of PE and PP had the higher density compared with seawater. Digestion experiments suggested that the density change of MPs was caused by the co-contribution of biofouling and inorganic minerals. The density increasing rates of PE and PP were 7.4%–12.6% and 12.1%–17.5%, respectively. Finally, we also estimated that the total number of MPs in the sediments of Bohai Sea was 534 trillion, which were significantly higher than that in global ocean surface, suggesting that the coastal sediments could be the sink of MPs.