Objective：To investigate the influence of lung inflation on MR perfusion imaging of human lung using an arterial spin labeling sequence called flow sensitive alternating inversion recovery (FAIR). Methods: Coronal perfusionweighted images were obtained at different respiratory phases from 10 healthy volunteers on a 1.5T whole body scanner (GE medical system) using FAIR sequence. The changes of tagging efficiency of pulmonary parenchyma (ΔSI %), pulmonary blood flow and area of the scanning slice of different respiratory phases were analyzed. Results: (1) Significant difference was found in ΔSI% between different respiratory phases（right lung P=0.021 5，left lung P=0.008 4）, with that at end expiration greater than that at end inspiration. (2) Significant difference was also found in pulmonary blood flow at different respiratory phases (right lung P=8.92×10-5，left lung P=0.000 2), with that at end expiration higher than that at end inspiration. (3) The areas of the scanning slice were also significantly different at different respiratory phases (right lung P=2.94×10-5，left lung P=0.000 5), with that at end inspiration larger than that at end expiration. Conclusion: Pulmonary blood flow during expiration is higher than that during inspiration, which might be due to the decreased lung volume and increased vascular density during expiration.