Biological control of plant diseases has been considered a viable alternative method to chemical control. These include Augmentation, Classical and Conservation biological control. There are various naturally occurring bio-control agents that aggressively attack on plant pathogens and suppress plant disease by different mode of actions. Bio-control agents comprise of multiple beneficial characters such as rhizosphere competence, antagonistic potential, and ability to produce antibiotics, lytic enzymes, computation for nutrient and niche. These biological control activities are exerted either directly through antagonism of soil-borne pathogens or indirectly by eliciting a plant-mediated resistance response. Understanding the mechanisms of biological control of plant diseases through the interactions between antagonists and pathogens may allow us to select and construct the more effective bio-control agents and to manipulate the soil environment to create conducive condition for successful bio-control. Some of the important factors that affect the efficacy of microbial bio-control agents in controlling plant diseases which should carefully be considered include biotic, a biotic, method of application, formulation of bio-control microorganisms and timing of application. Application and commercial use of biological control agents have been slow mainly due to their variable performances under different environmental conditions in the field. To overcome this problem and in order to take the bio-control technology to the field and improve the commercialization of bio-control, it is important to develop new formulations of bio-control microorganisms with higher degree of stability and survival. A significant improvement have been made in different aspects of biological control of plant diseases, but this area still need much more research for development of efficient bio-control, including development of novel formulations, understanding the impact of environmental factors on bio-control agents to improve its efficacy.