Abstract: Objective To investigate the relation between the change of perfusion parameter values and the formation of radiation pneumonitis (RP), with the technique of CT perfusion imaging (CTPI).Methods Sixty-four slices spiral CTPI were performed in 37 patients with thoracic tumor before and after irradiation with different doses in different phases.And varied perfusion parameters of region of interest (ROI) in lung tissue were assessed such as blood flow (BF), blood volume(BV), time to peak(TIP), permeability surface(PS), mean transit time (MTF) and relative values of these parameters were calculated, i.e, rBF, rBV, rTIP, rPS, rMTT.Results The perfusion parameters of non-tumor peripheral lung tissue before irradiation had no difference among lung or non-lung cancer and different gender as well.RP occurred in the cases with the remarkable boost in rBF, rBV and rPS (the value of F was 5.552,8.495 and 14.951 respectively, P<0.05), while RP would not happen if rBF significantly ascended only(F=9.146, P<0.05).Conclusion s The rule of perfusion change of lung tissue after irradiation could be detected with CTPI and it had been demonstrated that the application of CTPI in studying lung perfusion and RP was valuable.The parameters such as rBF, rBV and rPS had an important role in forecasting RP.