Purpose: The objective of this study is to investigate the influence of tumor size, location, and patient characteristics on the intrafractional target shift in lung stereotactic body radiation therapy (SBRT). Methods and Materials: Sixty-nine stage I lung cancer patients with partial upper body fixation who received SBRT in our center were included in this study. The patients had pre- and post-radiation therapy cone-beam computed tomography (CBCT) at each fraction for target alignment during radiation therapy setup. The 3-dimensional (3D), intrafractional shift and on-couch time for each fraction were recorded with soft tissue matching. Statistical correlations of these shifts with the characteristics of the tumor (volume, location in upper vs lower, central vs peripheral, right vs left, and internal motion) and the patient (age, sex, performance status, pulmonary function, body mass index, and on-couch time) were determined. Results: Overall, 470 intrafractional shifts were measured on the 69 patients. The median 3D shift was 2.4 mm (range, 0-15 mm) and the median on-couch time was 17 minutes (range, 10-55 minutes). On 55 occasions (11.7%), 3D shifts larger than 5 mm were observed. The tumor volume, tumor location (upper vs lower thorax), and the patient motion tendency were significantly associated with the probability of larger intrafractional shifts (3D shift >. 5 mm). Linear regression model analysis indicated male sex (P = .039), larger tumor volume (P = .019), and a motion tendency (P = .0006) are predictors for patients with a large intrafractional shift. No other factors were good predictors of large intrafractional target shifts in lung SBRT. Conclusions: In lung SBRT patients with large tumor size, being male and large motion tendency are prone to having a >. 5 mm intrafractional target shift determined from the CBCT soft tissue matching system. Prior knowledge of these factors would be beneficial in selecting a customized immobilization system for the lung SBRT patients with the specific conditions.