Oral bioavailability is defined as a fraction of the administered dose of a substance that eventutally being reached the central (blood) compartment after being released from food matrix in gastrointestinal tract, absorbed by intestinal epithelial cells and metabolized in liver; and the oral bioavailability is a fraction of an external exposure dose that having effects on human beings. The most useful approach to study the bioavailability of food contaminants is based on an in vitro digestion model that simulating human physiological conditions of gastrointestinal digestion, absorption, transport and so forth. Food ingredients and fat content, chemical and physical properties of substances, the concentration of nutrients other than targeted substances in foods, the nutritional status of the body and so forth will have a big influence on the bioavailability of the substance. The application of bioavailability in risk assessment of food contaminants is reviewed.