The microbial food web, which is comprised of phototrophic picoplankton, heterotrophic bacteria, virus and protozoa, is ubiquitous in marine waters. Members of the microbial food web may play important roles in the energy and nutrients cycle in the ocean. In microbial food webs, different types of interactions occur between microorganisms themselves and with microzooplankton. The functioning of the microbial food webs in different ecosystems has been well illustrated and explained. However, little has been known about the structure of the microbial food web since the first study in 2000 by Garrison et al. The abundances of different groups in the microbial food web are changing under different environments, but they mostly fall into a certain range: Ciliate 10 cell ml-1, nanoflagellates 103 cell ml-1, picoplankton 104 cell ml-1, cyanobacteria 104-5 cell ml-1, heterotrophic bacteria 106 cell ml-1, virus 107 particle ml-1. The optimum linear size ratio between predators and their optimal prey is 10:1, although this ratio is lower in field studies. For example, the ratio is 3:1 for nanoflagellates and 8:1 for ciliates. The ratio of picoplankton to nanoplankton abundance (Pico:Nano) is a scale-free index to study the microbial food web structure. The structure of the microbial food web depends, on one hand, on the type of environments and, on the other hand, on forcing factors exerting on these environments such as predation, phytoplankton and nutrient supplement. When studying the spatial variation of microbial food web structure, researchers usually use observation results of the abundance of microbial food web components from different water masses as an alternative plan. Regarding to the seasonal variation, the abundances and biomasses of microbial food web components are affected by the hydrological environment. Predation can affect the microbial food web structure through the following pathways: 1. Mesozooplankton feeding; 2. Mesozooplankton consumes microzooplankton and affecting lower trophic level through cascading predation effects; 3. Mesozooplankton could affect bacteria and other lower trophic level component by releasing dissolved organic matter and nutrients. Phytoplankton could release allelochemical matter and dissolved organic matter to change microbial food web dynamics. The concentration and variation of nutrients can have a direct or indirect impact on microbial food web structure.