Abstract: Histology method and electron microscopy technique were used to investigate the changes of respiratory tract lymphoid tissues in pig during growth. The result showed that tonsil and pharynx were the first place of lymphoid tissue aggregation for respiratory tract entering the body. Lymphoid tissue in the tonsil existed at birth without any apparent lymph nodule aggregations. Lymphoid tissue began to increase at 20 days, with clear lymph nodule aggregations. The lymph nodules increased and densely packed beneath the stratified squamous epithelium of tonsils at 120 days. The well-developed lymph nodules were present with germinal center. A number of intraepithelial lymphocytes were located between the stratified squamous epithelium of tonsil. The bifurcation of trachea is the second place of lymphoid tissue aggregation for respiratory tract entering the body. The apparent lymphoid tissue in the adventitia of trachea bifurcation linked with tracheobronchial lymph nodule at birth, whereas they divided into two parts at 20 days: the packed lymphoid tissue in the adventitia of trachea bifurcation and tracheobronchial lymph nodule. The trachea bifurcation was rich with lymphoid tissue with increased intraepithelial lymphocytes at 120 days. More lymphocytes were located in the laminal propria of branches and bronchioles in the lung. The number of plasma cells increased and few intraepithelial lymphocytes existed between the columnar epithelium. Our results showed that the respiratory tract of pig was the effective induction sites and effective sites of mucosal immune. The local immunity in the respiratory tract can be enhanced by the nasal immunity in the piglet.