Objective To explore the roles of antigen presenting cells (APC) in the lung of chronic inflammation by investigating the effects of smoking on the expression of B7-1 /B7-2 and its associated ligand in the lung tissue of rats．Methods Wistar rats ( n = 30 ) were randomly divided into non smoking group and smoking 6 weeks group，12weeks group，the number of each group was ten． Immunohistochemistry was used to semi-quantitatively analysis the changes of B7-1 /B7-2 and its associated ligands expression on the cytomembrane of chronic inflammation cells which were on the pulmonary interstitium around airways of rats． Results The expression of B7-1，B7-2，CD28 and CTLA-4 in the lung tissue of the smoking 6 weeks and 12 weeks groups' rats were higher than those of nonsmoking group ( P ＜ 0. 01) ，with the smoking 12 weeks group higher than the smoking 6 weeks group ( P ＜ 0. 01) ． Their expressions were increased in time-dependent manner． Conclusions Smoking can induce increasing expression of B7-1 / B7-2 and its related ligands in the lung tissue of rats; the APC may play an important role in the process of chronic pulmonary inflammation．