The statistical data of tuberculosis (TB) cases show seasonal fluctuations in many countries. A TB model incorporating seasonality is developed and the basic reproduction ratio is defined. It is shown that the disease-free equilibrium is globally asymptotically stable and the disease eventually disappears if the basic reproduction ratio is less than one; and there exists at least one positive periodic solution and the disease is uniformly persistent if this ratio is greater than one. Numerical simulations indicate that in the latter case there may be a unique positive periodic solution which is globally asymptotically stable. Parameter values of the model are estimated according to demographic and epidemiological data in China. The simulation results are in good accordance with the seasonal variation of the reported cases of active TB in China. This talk is based on a joint work with Drs. Luju Liu and Yicang Zhou.