The attempt of this lecture is try to understand the biocontrol mechanism of biological agents, such as bacteria, fungi, viruses and nematodes against plant disease to reduce disease damages or infection potential of pathogens from cellular or molecular level. Knowing the interaction between biocontrol agent and pathogen on the host plant or in environment is another objective of this lecture. The application of biocontrol agents and genetic engineering of a transgenic agent to improve or enhance its biocontrol effectiveness or to develop the transgenic resistant variety would be brought for detail discussion.