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In recent decades, the prevalence of asthma over the world has risen steadily especially in high-income countries.1 The underlying mechanisms for this phenomenon remain largely unknow, but environmental factors are thought to play an important role. Some epidemiology studies2-8 suggested that the increase in asthma inversely correlated with a steady decline exposition to some human diseases, such as tuberculosis and influenza. One of the general features of these infectious agents is that they induce characteristic Th1 type immune response which suppresses the Th2 response predominate in individuals suffering from asthma. Bacille Calmette-Guerin(BCG) is a strong inducer of Th1 type immune response. This review will focus on recent findings related to the interaction between BCG and asthma, and the main mechanisms of BCG in modulating asthma development.